Because it costs money to prevent it. And until perceived lost revenue exceeds the cost of a solution, they won't do anything about it.
I had a conversation a few years ago in the Fort lobby with a Disney manager regarding the throwaway room issue. Their perception was although it violated the rules, it didn't happen frequently enough to do anything about it. They get paid for their campsite so they don't care about the theft of the other services. In the Throwaway thread, I laughed when I read someone say the Fort was full and they had to reserve a throwaway Value room at All Stars. But All Star management doesn't care because they got paid too.
Locals using the pool are the same. Until enough paying Fort guests complain longly and loudly enough, management won't do anything about it. They can post an extra CM at the gate with a MB scanner and require swimmers to have their MB's but that requires extra staffing (money). IIRC the pool at WL has a device/scanner that reads your MB to open the gate (a system which costs money). Halloween is the same way when locals overwhelm the Fort.
PS - We are really lucky we have the Fort to camp at while at WDW. Disneyland has no similar on-property offering and Disneyland Paris has Fort Davey Crockett which is just cabins only (no campsites at all). So we are an exception to the normal mode of business.