I think LisaS' explanation sounds the most plausible to me, that they simply have a true/false flag on the use year that you'd already used one-time-use points. I'm betting the software came first, then the rule. They didn't want to create the logic to keep track of how many of 24 points had been used.
I personally don't think it has anything to do with the one transfer per year logic, as you'd still be able to do a transfer with another user even if you'd purchased one-time-use points, and vice versa. Nothing in the rules indicate that a purchase of one-time-use points constitutes the use of your annual transfer. Also, the one transfer per use year restriction is at the contract level. If a member has three contracts under their membership, they can have three transfers per use year; one for each contract. For one-time-use points, the restriction is one per year per membership, not contract. So that same member with three contracts would still only be able to purchase one-time-use points once per year. (This may be one of very few tangible perks of having multiple member numbers to juggle.)