There's a registration option to pre-register and pay on-site if you make it. So, yes, you can sign up (and be on the list), not pay, and then not show up. No need to cancel, although it is courteous to do so, in case the event is full and someone is hoping for a spot to open (at least if it's before the deadline).

Of course, we'd like to have everyone who registers show up - but we understand sometimes it just doesn't work out.

This isn't annoying...