Here's my flip on this (True Story):
Some years ago I pulled into a Rite-Aid on a rainy night to get some cold meds because I had the flu. This was an older parking lot where the lines were worn down and tough to see, not to mention the pavement was wet. There were no handicap signs or other markers.
Even though I couldn't see designated parking spaces, I parked what I thought was a safe distance away from the entrance because I consider it common sense to simply respect designated handicapped spaces even if they aren't clearly marked. For me, this is just something 'one does' without debating it.
I went in, and got my meds and had to wait in line for 15-minutes because every moron and slow poke in the known universe was in that same line that evening. So, not only did a feel like crap, and it was a crappy evening, but I had to wait for all that time. When I finally got back out to my car there was a nasty note on the windshield in a plastic bag calling me lots of names and saying things like 'you need to thank god you aren't handicapped', blah, blah, blah. Not having a clue what was going on I pulled out about 20 feet, got out, and in the drizzle walked around the parking spot using my headlights as a guide.
Sure enough, there were a few specs of faded blue paint that a forensics team might determine was an ancient handicapped spot when Nixon was president.
Had I caught the guy that night in the act of writing that note I would have likely shoved it down his throat, handicapped or not.