$10 free from UB for trying to deposit with a US credit card

[synth_floyd]

I tried to make a deposit with a US credit card about a week ago and it was rejected (though they told me it had been "pre-approved" but never actually put any money in my account). I didn't e-mail support about it and just ignored the fact (though I checked to make sure nothing appeared on my credit card statement. Nothing did) and then today got an e-mail that a "no-strings attached" $10 had been credited to my account. I checked and it was in my account. I assume people could knowingly try to make a deposit with a card that won't work just to get the free $10.