I'm not sure about police not enforcing in private parking lots - DH and I once stayed in a motel, the room we were in was for disabled (last one left) but the clerk said we could NOT park in the HP spot right outside the room. He said the police came through the parking lot and would ticket us. This was before DH got the tag.
We also learned, after DH got the tag (stroke survivor, problems walking) that where there are meters on the street, you can park at any of them without paying. You notice HP spots on the street do not have meters. This was in NH, not sure about other states.