I guess it is again a cultural thing. It is not generally well seen to leave a job at the first opportunity, after a short period. It's a matter of trust and commitment, I guess (and also of government paperwork, around here that part is costly).
If I join a company, I know I am expected to stay a certain period of time, at least until the company is earning money with me. Depending on the job, that is estimated to take around 6 months. If I leave after a short time, the company probably spent more money on me than the money it earned from my work.
It may not be the case where you live, but here the IT world is a small place, and the chance of applying again to a job in the same company is not as small as one would like to believe. Quitting your job in good terms will most surely allow you to come back if the opportunity arises. Quitting after a short period of time will probably not be "good terms" for your boss.
Even if you hate your job, you will find out that connections matter. I worked 5 years for a big company, and then moved to a company leaded by a former boss. Now I am working at another big company, thanks to recommendations from a former colleague. If things go wrong with my current job, I know I can talk with my former bosses and see if they have any positions open.
Also, people in management positions know each other, specially if they work in the same city, and even more if they have many years of experience. Work professionally, and your ex boss will recommend you (if he's not an asshole) if he gets asked you by a friend in the company you are trying to join. The opposite holds, of course.
I don't want to imply that you should bend over to the company's requirements, but one should know how to move around in the job market :)