By law the hotel has to register your arrival online, as a foreigner. Obviously this is easily skirted by having another person do the check in and you arrivin

By law the hotel has to register your arrival online, as a foreigner. 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.