I'll just say that in PA, you are required to register your ATV/UTV.  and in order to register, you are required to show proof that you have liability insurance for your ATV/UTV. 

if you run your ATV/UTV only on your own property, you can be exempt from the registration and the insurance.  However, to run it on property other than your own, you need that insurance. 

That is where I am coming from - if the state of PA requires it for me, then I guess the owner above would be within his rights to require it also.

I am assuming that PA is not the only state to require liability insurance for ATV/UTV's.