We just got off Pearl and I was wondering if anyone has ever actually been charged for a missing pool towel. What a outdated system and definitely seems like a hollow threat. At least on RCL they use your room key to check towels in/out, but its very easy to cheat that system as well, if you are off on your towel count they will usually just trust you if you say you returned all your towels. No one is stealing towels off the ship in any quantities that matters anyways. They are just concerned about people returning towels for cleaning, and keeping people from using towels to reserve chairs for hours that they never use.
We went to a Jamaican resort a few years ago and they had towel cards. Each guest got one. You could get more than 1 per guest if you wanted. You could exchange the card for a dry towel, and then return the wet towel for either a dry one, or a card back. On checkout day you had to return as many cards as they gave you. Pretty low-tech but it worked. At Legoland they have a fancy towel machine that uses a chip in each towel, you scan your ke4y, take towels from the cabinet, and it tracks how many towels you took. The return chute scans the tags as the towels get dropped in. The machine's door was hanging wide open so they have some logistical problems there but it was pretty neat tech.