a) may be prohibitively expensive.
b) may very well be highly regulated by law (and you thought guns were regulated).
c) might very well not change one's own present, only the future of an alternate timeline. This greatly reduces any incentive for attempting to alter the course of history, because it would have no effect on your own history; and would only affect one of a potentially infinite number of timelines.
No way. There's too much money to be made in time travel. It would be far more difficult to regulate than the drug trade. Besides, if you got caught breaking the rules, you could just go back and make the necessary correction so you wouldn't get caught. No problem. Time travel will always be unregulated.