The CMs never ever try to make the resort monorail for resort guests only. And I think this is for several reasons really:
1. There's just not time to check everyone's resort card prior to entering the monorail- there would be too much of a hold up, and place a lot of responsibility on individual cast members to try to discern what was a valid card and what wasn't, and try to ask certain people to leave, etc.
2. There are lots of reasons why someone who isn't a registered guest at a monorail resort would want to be on the monorail- just to visit the resort, contemplating a future stay while on their way out of the Magic Kingdom- restaurant ressies- DVC member pool hopping- official scavenger hunts for pins, the list goes on and on.
3. It's my belief, and I know it's just my opinion, that Disney does want people to explore and see these resorts and make use of some of the paid amenities at these resorts which don't require being a resort guest, such as the Grand Floridian Spa, the racquet club and the water sports center at the Contemporary, all the restaurants, all the gift shops, etc. etc., as well as seeing the places and wanting to make a reservation in the future. It's my opinion that Disney both a. wants people to see and make use of these resorts, and b. knows that trying to make that monorail "resort guests only" would take much more human resources, time, and cost than it's probably worth.
As for the early entry, if offsite guests really are getting off either of the monorails during early entry and just going right through the turnstyles, then that is an issue, and they should be checking every single resort card at the turnstyles for that.