Tacky as hell and a good way to lose some friends. So if they died in a car crash on the way to the wedding are the kids supposed to pay? stuff happens. Normal people would expect some no shows. The freaking universe does not revolve around "look at me on my day" brides.
We could argue this till kingdom come, but I don’t think either side lost much in terms of ‘friends.’ Both have shown poor form.
The guests weren’t killed on their way to the wedding; had they been, it’s unlikely a bill to ‘send a message’ would have been sent. They simply didn’t plan well enough and when their inadequate plan failed, they compounded the failure by not informing the hosts. Tacky all the way around.
Yeah, stuff happens, but there’s still a basic etiquette that should be followed. We can’t on the one hand talk about the decline of civility and manners, and on the other give kudos to and make excuses for folks who are rude enough to fail to inform their host that they won’t be able to make the occasion that they had to know was costing the hosts.
By the way, did you know that in Japan, guests pay to attend weddings? Something to the tune of hundreds of dollars.