Because it is less forgiving to a new rider, and the mistakes a new rider does make. I will give a few examples.
Keep in mind all the GSXR's whether it is 600cc,750cc,1000cc, they are all race bikes that have lights installed to make them street legal. Being a race bike, they are tuned and designed to respond to the most minute inputs, which in turn, makes them a dream to drive for an "Experienced" rider. However, in the hands of a new rider, mistakes in input, which ALL new riders make, I do not care how responsible or godly talented someone is, is magnified greatly.
The brakes on these bikes are amazing. Anything more than a gentle tap on the rear brakes and you can lock them up quite easily(they are designed for use on a track with hot tires and optimal road conditions, which you will not have on the street). Locking up the rear brakes as a newbie rider on a race bike usually ends up with the rider high siding. And taking a dump into the road or another vehicle is the last thing you want to do.
Having no experience on the road with a bike, you do not yet know all the situations you can face on a daily bases.
Say you are going down the road and someone in opposing traffic does not see you and turns left cutting right in front of you're path(one of the most common causes of a moto crash). You have been riding for a month or two and think you got the hang of controlling you're new bike, but when this situation pops up, 8/10 times, being a new rider you "Will" panic and grab too much brakes(simply because it is soo easy to do so on these bikes, and if you are in a panic and inexperienced, you most likely will).
Grab too much front brake brakes in this situation, and you are launched forward as the bike bucks you off and flips. Grab too much rear brakes and the back wheel locks, and with a new rider with very little experience, you will most likely high side from not keeping the bike straight.
That is just one of many situations you can face on a daily bases and a very very common one.
I would say the throttle control can also bite you in the ass, 2-8.5krpm and you may think, hey, this is manageable and you may start to feel comfortable with it. However say you hit a bump in a turn or something that may make you grab a handful of throttle whether you want to or not(being a new rider, most likely you will be gripping the throttle more that you should, and with a bump, you will end up turning it more than a 1/4 inch). This happens and all of a sudden you hit that powerband for the first time(it is a little higher up on the 600 and can surprise you even fi you know it is there), if you are in the turn with little experience, it will not end good. Even in a straight away a pothole sending a newbie riders deathgrip straight in the powerband can send you screaming into the vehicle in front of you or causing you to freak out and turn the bars slightly and spill in the road.
Yeah alot of people say you can take it easy, and "Respect" the bike and you can be alright, but on the street, conditions are ALWAYS against you. Whether it is the cagers not seeing you, debris in a turn or in the road, oil or slick conditions, and even more so when you are still learning to ride a bike.
Riding a motorcycle on the street is constantly rolling dice, and you always want to put the odds more and more in you're favor. Having good control of you're bike is #1 first thing you can do to have things more in you're favor, and if you are a new rider on a race bike, good control you do not have.
Alot of the things on the road are outside of you're control, so you want to put yourself in the most favorable condition that you can. That is why the arguement that "Respecting the Power" of you're bike and "Taking it Easy" are null and void.
I have only given a couple examples of how these bikes can really bite a newbie in the ass, but will list more if you want.
Now if you get a sport/touring bike, even something with 750cc, and spend a season or two on it getting familiar with balance, control, and drilling emergency manuevers into you're body so when the time comes you act accordingly, future panic situations on an unforgiving race bike will not end with you in the hospital or worse.
A more forgiving bike requires alot more input to control and is alot nicer to rookie mistakes. You will have much more room for error in The above examples I gave if you're bike is not a race bike.
Also another benefit is you can actually ride the bike more to it's full potential if it is more newbie friendly, which is alot more fun than just trying to survive on something you will never fully tap into on the street.
You will also learn much faster on a vehicle that you can tap more into, this is undeniable, 1 month on a 500cc, and you will be better than you would with 6months on a GSX-R600 as you're first bike.
And after a season or two if you bought used, you can sell it for basicly what you got it for, and then buy you're gixxer and be a much better rider because of it. Also with race bikes being for "Experienced" riders, you will have ALOT more fun on the bike when you have a good foundation built riding bikes, because in the hands of an experienced rider, they are amazing machines and alot of fun. In the hands of a newbie, they do not last long, or are never truelly tapped into and enjoyed.
Do not get me started on how much more it will cost you to drop a gixxer as opposed to dropping a 500cc-650cc without fairings.
This is the first bike I owned in my name, and I learned the most on it. Also being designed for touring, it was an extremely comfortable ride for long trips as well as in town.
Most of the people on here with more experience than me would recommend an sv650, or a gs500. I have not riden either, but have heard nothing but great things about both machines for beginner use, especially the Sv650.
As someone greater than me once said, you do not know what you do not yet know.
