Is there a catch to the Amazon credit card offer?

Have any of you signed up for it? I’m on a mission to find some good, heavy duty gloves that will fit me. I found some hot pink ones : ) ones for about $14.00 after shipping. There is an offer for $30 off “instantly” if I sign up for an Amazon Visa card. I clicked to add to my cart, and according to the calculator off to the side, if I sign up, the gloves won’t cost me anything, and I’ll still have a $21 credit. I’d buy a poster of the solar system with that $21, and get IT for nothing. And then cancel the card!
Is there a catch?