I could not give a detectorist permission to detect my property because: he might leave the gate open to the main yard and let out my tom cat who would then proceed to impregnate all the cats in a ten mile radius, then dig holes in my yard and then artfully manage to twist his ankle in one of them, then, my dog seeing all the digging going on, soon realizes he's getting lessons from an expert, then suddenly gets better at digging holes under the aforementioned gate and fence, then, knowing my luck, his batteries would die on his machine and i'd feel sorry for the guy and go around the house emptying all the remotes of their double a's. then, my neighbors would, without fail, wait until the guy leaves and ask to borrow my new double a battery operated weedeater. then, he could cut into my buried cell phone cable... i'd just never hear the end of it. besides, he might just be a thief casing my place. somewhere, a clump of grass is screaming because of him.
hello EL! great topic. what i have done is kind of turned the topic around a bit to illustrate how some land owners think about us detectorists. there really are people who think and assume the worst when you approach them about detecting their property. as del mentioned, violation of privacy is probably the number one reason. personal liability also weighs in heavily. then, there are those who don't want history recovered at all and are content to let it degrade in the ground for at least another generation. personally, i wouldn't have a problem letting someone detect my yard - i don't have much grass anyway.
i know, i know.... there's no such thing as a buried cell phone cable around a house... but i heard that one last year! thanks for a great topic.