>I use X every time and only once was I stopped (Home Despot).

Just like sometimes you get a clerk who won't take a $2 bill. Nothing will ever trump human ignorance.

X is a legal signature, they can't choose to not accept it.

They can refuse your transaction for whatever reason they want, it's their business. If you give off any vibe that the card might be stolen, then a business can turn you down whether or not what you were doing was technically correct.