If you think about it logically, why should they let you? They have nothing to gain. You're asking them to let you dig holes in their yard while looking for valuables that technically belong to them. The insurance liability aspect is another factor.
My attitude is that it is a law of numbers. It doesn't hurt to ask - the worst is that they say "no". Some will say yes, most won't.
I chat about the history of the area, my hobby, etc. I describe how I retrieve an object and offer to demonstrate as to alleviate visions of ugly holes or scars. ( call what you're looking for "artifacts ' or "objects", not "coins", etc).
I also offer to put everything I find in a pile so they can take a look (you be the judge as to what actually goes into the pile - lol). Most don't care if they've given permission already. I find that if they give permission, their attitude is "have fun".
I got open permission to hunt a farm here in Ct. The owner had said one time he found an IH cent on the farm and wanted to attach it to his Indian motorcycle but lost the coin. I told him my goal was to find him another one, which I did. He then gave me free reign on the farm - anywhere, any time.
Attitude is everything, approach is important.
Bruce
