Apparently not many. O'Brien says "There probably hasn't been a case dealing with this in a hundred years."
Also, the sentence was very light. Julian wasn't punished at all, the person responsible was. And Julian's mother was left alone as well. Julian's dad only got 2 years in a walled resort, where he'll have to wear an anklet, and build garden sheds.
I assume the blanket ban exists for the great potential of abuse that exists. By outlawing the practice except for necessities, the government has the legal power to go after those that would abuse the science. The potential for abuse also exists within the individual who has been modified. It would give them great power over others, and a potentially corrupting existence.
If it was totally legal, everyone would eventually have to be genetically enhanced to compete. A new race of human would replace the natural one. There's no predicting what this society would turn into.