There are bands that are buddies hanging out and there are bands that are businesses. If the band runs as a business, I would assume they thought it was just understood that if a player can't make it, the band gets a sub. Probably didn't occur to them that it would be an issue to you. I agree it's a little rude just to inform you of it rather than talk to you about it beforehand. Good practice, if you can, would be to have a friend who subs for you. Then the band has you to thank for saving them the hassle and you can be reasonably confident your friend won't let them replace you with him. But ultimately, if you're worried about being replaced, the best insurance is to work on your playing so that they can't find anyone who can replace what you do.