It's not a State occasion, William isn't heir to the throne, just a prince so there's no need for the world to be at his wedding. As far as we've read and understood William wants what for the Royals is at least a quiet wedding.
If it's true he's not invited it's not a snub at all. No point is getting your knickers in a twist as whoever was in the Oval office wouldn't have been invited, nothing to do with personalities.
I don't think it is so much that he is "just" a Prince or anyone getting bent over Obama not being able to attend as the surprise that one of the well liked royal personalities is getting married and an arrogant American politician is not getting to share the spotlight. My .02 only and it doesn't count.