I think that the issue comes in when people have the option of both plain sight and searching. For example if an electrician is paid by the local PD to report drug paraphernalia they now have an incentive on future calls to poke around where they don't strictly need to be. How do you know whether they conducted an illegal search (as an agent of the government since they are paid if they find something) vs. saw it in the regular course of their duties? To me that is the key. The person is already in a somewhat privileged position, not just standing on a street corner with their eyes open. The payment creates a conflict of interest, thereby making anything they find suspect.