15 years ago a foreigner couldn't even get a hotel room in Thailand without handing over passport information for each guest. I can't imagine it's changed much since then. If the Thai police want to track you down it wouldn't be very hard.
Obviously this is easily skirted by having another person do the check in and you arriving later. I've also stayed in some guest houses in Thailand last month and they did not register me, but that doesn't mean they should have done so.
If they don't, technically you're on the hook for not registering.