The rental contract for park rental
ECVs and all the off-site companies I am aware of forbid passengers. Most also specify it can't be operated by anyone under 18 (Randy's will sometimes make exceptions for older teens).
Disney can stop people from doing something that is causing a dangerous situation for themselves or others. It would not matter where the
ECV ís from.
I think though it has become so prevalent that little can be done about it. I have seen CMs stop someone from having their child ride on both park rental and other ECVs; the people would often stop just long enough for the CM to get out of sight.
ECV accidents would normally be covered by the owner's homeowner's insurance. Since the owner's manuals warn against having passengers, it's possible the insurance could refuse to cover an accident with a passenger.
I was told by a CM at OKW in Bell Services in April that it was still forbidden, but front line CMs had been told you contact Security and let them handle it.
First of all, so that there would be a report and second because guests had increasingly been getting nasty when told their kids could not ride.
In the case at OKW, we saw a woman (probably a Grandma) getting on the buses and using an ECV in the parks. At the resort though, 3 preteen girls we had seen with her would take turns riding it at high speed - usually 2 at a time, but we saw them several times with all 3.
The Bell Services CM saw them almost run into me, which was what started the conversation. He said Security had already talked to them before.