Because many stars think they can do anything and get by with it and lot's of them do get by with stuff that we'd be in jail etc over. It gives them totally free publicity and the main reason is because there's so many people that thrive off of gossip and scandals that it's made it a easy way for stars to get more famous.
Some just make mistakes when they are young, others do it just to further their career - or in some cases start their career. They can do the whole "My Story" thing in the newspapers, write a book (or more likely get someone else to write a book) and worst case end up on some crappy reality tv show. All of which means more money in the bank.
If your trying to get noticed and make a name for yourself then any sort of publicity is good publicity. Web publishers can learn something from the likes or Paris. How about that.
some are just notorious for bad life choices but like has been said...any publicity is good publicity!