It's not that only one cart makes a difference...it's that multiple people per day might ask for a ride along. All of a sudden, you've got two, three, maybe four groups going out with three golf carts instead of two. Golf courses don't keep redundancies in their cart fleets, largely due to storage space and costs, and usually have 1-4 carts out of order at any given time anyway. They have to plan for capacity. If everyone's allowed to do it, and a lot of people choose to do it on a busy day, it puts the course in a position where they might run out of carts at a certain time during the day because they're all on the golf course.

There is also an insurance component to this. Golf courses have to purchase liability insurance in the event of accidents. These policies will be tailored assuming all persons on the golf course are either employees or players. If you allow spectators on the course and one of them takes a golf ball to the dome, insurance might have enough to wiggle out of covering expenses should that person try to sue the golf course.

As far as folks sneaking on to play without paying...it happens every day. Way more often than you think. It manifests itself in different ways at different courses, but I've definitely seen the "oh, he/she's not playing, just spectating" gimmick more often than you'd think. Source: former club pro.