Is it legal to accept a later offer (e.g., came after 15 April), despite having previously accepted an offer from a university before 15 April?
You ask whether it is "legal", but it is probably not a legal matter.  You can drop out of an academic program at any time with no legal consequences beyond paying the relevant fees.  Unless your acceptance means signing a contract which specifically requires something on your end, I don't see what you could possibly be legally held to.  I am not an attorney but rather a professor, and I can say that in practice no American graduate program would pursue legal recourse against a student in this way barring some truly bizarre, unique set of circumstances.

The relevant questions are rather whether it is ethical, whether it is polite, and whether it is in your own best interests.  The first question seems quite clear.  Are you really asking whether it is "okay" to rescind your word in a professional context?  I would hope that you know the answer.  

There may be some situations in which it is so much in your best interests to renege on an offer that it could be worth asking about the possibility of doing so.  Most graduate programs in particular are not insistent on keeping students who have decided that they don't want to stay there.  But if you're asking about accepting now with the explicit plan of reneging later: I don't really know what to say without it sounding like a lecture on adult responsibility.  This is not a specifically academic issue. I can only hope that your life up until now has given you some useful experience.