If the issue is use of the pool, then restrictions should be aimed at the pool, not at people who want to visit the Fort (or other resorts) for other reasons. Wristbands issued at the gate to the pool, along with periodic wristband checks looking for those who slip in behind registered guests, could be effective. I agree with PP that Disney doesn’t want to forgo revenue from gift shop sales (one of my favorite pairs of socks came from the trading post, bought when we weren’t staying at FW) or non-ADR food sales. Checking for resort reservations and ADRs at DS or any other Disney transportation depot (we rode the boat from MK, just to shop at the trading post and have a drink) isn’t reasonable, IMHO.
PS A similar discussion occurs periodically advocating that only registered guests and those with ADRs should be allowed to ride the Resort monorail.