I think if someone throws a "surprise wedding", it's because they don't think weddings are such a big deal. They just want to get married, and they want some of their besties around when they do it. In general, these things, when they are done, aren't "weddings" with all the weddingy accroutrements and paraphernalia. They tend to be arranged as get-togethers, which happen to have a little ceremony thrown in, with no expectations on the demands or guests other than to be happy for the couple. And usually, only people they know well and who they know will be happy for them would be there. I don't see how it is rude, any more than simply announcing an engagement at a dinner party is rude, or telling people that, by the way, you got married last week.