Why is the ouija board illegal?
In the USA the Oujia board is not illegal, and available in most stores like Walmart, Toysrus and other stores that carry board games.
Some cultures forbid their people to use them, and some other countries might have laws against the occult including Ouija boards.
In my experience they have brought trouble to a lot of people. I have done paranormal investigations for people who have used Ouija boards and shortly after had very negative paranormal experiences.
No one can say if there is a correlation between Ouija boards and entity intrusion, possession, or poltergeist activity, however there are plenty of testimony from people on the subject.