See my other response regarding this activity. I'm not an attorney, but there is a fine line between illegal and legal trespass. For instance, meter readers don't ask for permission. Utility repair crews or surveyors either, even when "trespassing" on property other that than for which they are engaged.
How come? Because states, communities, and courts realize that there exist occasions which require such unfettered access. Knocking on someone's door to verify information is not illegal, especially in performance of an activity for which you are licensed by the state.
Once again you and I seem to have little in common. I would never advocate for someone to do anything for which they felt unsafe or otherwise at risk of life or limb. That said, many are simply too afraid to be nosy and ask total strangers questions, either because they've never been trained to do so, or because they believe it to be none of their business. What are these people even doing in this business?
It is the nature of this job to be inquisitive, to ask pointed questions at times, to engage total strangers, even those who appear different from us.
Again, appraisers are such wussies. Asking permission to take pictures is merely a way of being polite, a practice that most people appreciate and are tolerant of.
If one finds much hostility doing such verifications, they should work on their people skills.
PS. 38 years in this business and I've never once heard of someone being charged, let alone convicted or fined for trespass when performing an appraisal. Yet this group of cackling hens worries about it as though it's a frequent occurrence. My advice is to worry about things that really do happen and can actually hurt your careers and business, like being found negligent in the verification of your comp data, for instance.