newsman, you've got some good answers so far. But let me tell that often-time, when you hear a land-owner or old-town demolition contractor cite such a reason for his "no", it's not really the reason for his answer. It's just the easy go-to answer to tell you "no", when .... truth be told, they just "don't want to be bothered". Because even if it WERE true that you could have some way to fully protect him (that WAS worth the paper it was written on, and could not be assailed by a lawyer if you got hurt), the truth is, that to mentally cogitate this, on the part of the person your asking, takes his time and energy to consider, read your cert's of ins, file, open and close the gate, etc... blah blah.
I own a company, and as such, have a $1,000,000 liability policy for our street sweepers. In the past, when hearing the "No" because of "liability...", I cheerfully told the persons: "No problem! I have a $1m policy through my company, and this will simply be a 'job' as I'll call it, and I'll have a cert. evidencing proof on your desk via fax in 5 minutes!" To which the person saying no STILL tells me to "scram". It is then that I begin to realize, that they simply want to say "no", as not to "be bothered". And it really doesn't matter what efforts I take to remedy their concerns. Any such "remedy" just means they'll have to wonder where you are, what you're finding, if they'll have to process paperwork, etc... Why should they be bothered? Afterall, they want to go home at 5pm and have a beer and watch TV. Not have "1 more person" they need to be thinking about.
In the same way that we close the door on door-to-door solicitors, or hang up on phone solicitors: NOT because the product isn't great, or cheaper than what we're already using or buying, right? But SIMPLY because we don't want to be bothered, and have to go through the hassles of comparing their product, with the thing we currently use and buy.