I will add the BJCP documents do suggest one sign up for the tasting before taking the online exam.
Given the current demand, it's silly to take the qualifier exam until you have a slot reserved for the tasting exam.
Possibly the new exam structure will discourage people from backing out. If you've already got skin in the game, even if it's $10, and a time limit of 1 year before you have to start over from scratch, you might be more motivated to show up for the tasting exam.
I can't take people bailing out the exam that personally. For some folks it's change of interests, attacks of nerves or lack of preparation - if they're not prepared, why waste everyone's time by forcing them to write a crummy exam.
Other people get sick or have Serious Stuff crop up. Given the lead time to get an exam seat, you can't predict what's going to happen to you in the meantime.
What is polite, though, is to RSVP. People who know they're going to have to bail should give at least 2 weeks notice, preferably a month.
Let's just hope that the new exam structure gets more graders back in the system, so the number of scheduled exams can be increased. Right now, everyone's watching and waiting to see how the new exam structure affects things.