Their partner sees "XYZ.com" on the bill and goes to look it up. "Why are you buying porn?" "Oh that wasn't me! Someone must have stolen our card" and they submit a chargeback.
I'm sure regret is also a part of it as well. "Why did I spend that money?" And of course, outright theft of services, where they watch/download whatever it is they're interested in and then cancel the charge.
