First, I really didn't like the tone of the card company. They're selling someone else's work (whether legal or not), and then were condescending toward the person who made it. They say they're just trying to honor Banksy, but they paint him as an out-of-touch rich asshole.
Second, it's hard to know for sure, but I think this analysis misses the bigger point... I feel like opening a small shop that isn't real is very Banksy-esque, and likely is more of a statement on trademark than him actually trying to enforce a trademark. This is clearly just an exhibit + marketing. (It's a glass storefront, after all)