I have found that the parking guards at the car parks for resorts seem to be quite happy to let you in to park if you mumble something to them in a really strong regional british accent. Geordie or Liverpudlian is good. The more inaudible you can make your accent sound, the quicker you'll be allowed in.
The official Walt Disney World line though is that only hotel guests and people with ADR's or visiting the hotels should park at hotel car parks, and those car parks should not be used as free theme park car parks.
Its a tough one. If I had paid £250 a night to stay at say a deluxe resort, and when I got back to the hotel I couldn't find a space to park because lots of people had parked up and then gone off for a jolly in the parks, I know that I'd go ballistic.
But on the other side of the coin, if I was going for a night at the boardwalk, spending money there, having dinner, etc, then I wouldn't want to be parking at the public car park, highlighted in the picture below. That car park is so far away, across a busy road, and has these big cargo trucks parked on it, like an overnight truck stop. I wouldn't feel safe walking my family back to it late at night.