Is it considered rude to snap your fingers to someone to get their attention?

That is considered extremely rude in the United States. We interpret it a sign of open disrespect: that you believe the person you’re snapping at to be your social inferior. Generally speaking, we only snap our fingers to get the attention of animals and perhaps children, so that is the context within which we’ll interpret a snap. If you snap to get the attention of an employee at a business there is a very good chance they’ll ignore you and pretend they can’t see or hear you. Your service may be discontinued, and if not, then it will most certainly suffer in quality. Snapping to get the attention of a random stranger is an _excellent_ way to provoke a confrontation. Maybe you’ll be ignored, but the chances are good someone may take it as a sign of deep disrespect. Racial, class, and cultural differences play a role, too. While almost all Americans won’t like you doing this, if you’re a clearly affluent person snapping at a less affluent person it will make it worse. If you’re a white person snapping your fingers at a black person then it will likely be interpreted within the context of America’s racial troubles. It’s a really, really, really bad idea.