I think the real problem is that the brides expected their wedding to be an "investment", with a net profit. Otherwise, even if a guest gives a gift they feel is inappropriately cheap, it can be laughed off, right? "Oh, that Frank, he'll squeeze a nickel until the beaver screams*, won't he?"
Instead, they've clearly lost sight of why they're hosting - that is, spending their own money so that their friends can be with them during a day when good feelings should be overflowing. Instead, it appears they viewed their wedding as a stage production, and one of the audience didn't pay for his ticket in full, so they're sending an invoice. This is really a terribly sad thing for them.
*This is in reference to the Canadian nickel, of course.