To answer the question (in case it is not a joke).
Everything your mother did is legal. Social services will laugh if you call them. They have real problems to deal with and kids who really need their help, not just kids who are upset because they can't do whatever they want.
The issue here is that you are not respecting or appreciating your parents who obviously are not perfect, but also obviously care enough to not have strangled you when you completely embarrassed them.
Look at it from your mothers point of view. It is not a good thing for kids to throw food at strangers. That isn't fair. I would be horrified if my kids did this. Any one of us would have done the same thing your mother did. You will do the same thing to your kids if they act this way.
Your parents not only have a right to ground you. They have a responsibility to ground you. You can't run around disrespecting everyone around you.