I understand that, and it's a risk I'm willing to take. I'd rather find out that a person is an ass before I give them my money than after. There's always another store, another car dealer, another hotel, another vehicle rental chain. It's likely that the after-sales service will match, or be worse than, the pre-sales service. I'd rather not give them my money at all than have to fight for recompense after the fact.

I don't think that's a reasonable attitude. Ignoring the signature line, even if common, puts a company at much greater risk of eating any fraud committed with it. If an unsigned stolen credit card is recovered, its owner has a very easy claim against any stores that accepted the thief's signature: "obviously they didn't check the back of the card, now did they?" It feels a little entitled to insist that a merchant who wouldn't accept an unsigned card is a bad vendor giving bad service.

An opinion doesn't need to be reasonable. I'm aware it's not the correct way to do things, but it provides me and my money with a little more protection IMHO. No card thief will hand over a card with "Check I.D." written in the signature strip to a clerk, just in case it's one that will ask. If the clerk does as asked, they verify everything is in order. If they process it anyway, it falls back to the remainder of the fraud protections available; heuristic monitoring at the bank / card processor, regularly checking bank statements etc.

One thing I'd like to challenge is that the card isn't valid unless it is signed; Technically the valid signatures for my cards are "Check I.D."; the act of signing makes the card valid, just as if you sign a contract with "Mickey Mouse" you are still contractually bound by its terms. You are treated as assuming the persona of "Mickey Mouse" or "Check I.D." while signing; it's the act that is important.

Just don't tell the card thief that ;)