That’s because that’s how he has treated them. Lying about them and what they are doing is nasty. That’s what Banksy did.
Maybe it's moot, but... yes, Banksy makes a lot of money off his work sometimes.
But there's some things worth mentioning:
1. He also spends a lot of money on his work (look at Dismaland, which was only $1/ticket). 2. Much of his high-priced work is sold by private collectors and he doesn't get the money 3. Or, the one he did sell directly was art itself, and the sale was part of the art (it got shredded) 4. And lastly, he donates it. Like all the profits from this store are getting donated.
I wouldn't necessarily confuse "there's a lot of money surrounding Banksy" with "Banksy is rich".
What the card company is doing is to take advantage of his desire to be anonymous and make money from his work.
The other point is that Banksy is lying about this small business and what they are doing. This is an objective fact. He claims they sued him. They didn’t. He claims they are a big corporation. They aren’t.
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)