This is why I hate societal shaming and vigilanteism. I know we're talking about people with handicapped tags, and thankfully, for the most part, I think the system works pretty well -- I bet the scammers and jerks are few. And yes it can be jerky if someone borrows a family member's tag or parks in the space without a tag, but in the end it's a parking space. There are so many other things to be angry at than whether or not people who are "properly ill" are able to park a little closer to the door, or have to wait for proper parking to open up.
I have the same problem when people make assumptions in general. For instance, shame a woman who "looks pregnant" for smoking or purchasing alcohol. I had one friend, for instance, who proudly claimed to have shamed a pregnant-looking woman for buying beer while cashing her out at her workplace. Can you imagine the nerve? There are dozens of scenarios I can think of where a pregnant-looking woman would buy beer, including her possibly not being pregnant!
Assumptions are a dangerous thing, and I certainly wish people would consider more than just the worst case scenario when throwing down for such relatively small things -- why die on such a small hill?