Actually if you cancel within 60 days of the fee being posted they credit your account. And if you've already paid the fee when you cancel your account they send a refund check. I just did this a couple months ago (but on a card that gave me 15,000 miles). Obviously they've caught on to this scheme, hence the 6-month requirement. It's still a good deal though. $60 for 20,000 miles is a steal.

Also, the reason for my "how can they do this?" question was simply because United and BankOne are completely separate entities and I didn't understand how BankOne could just call up United and tell them to debit your account. Every single time I've had any problem with my Mileage Plus account I always find that BankOne blames United and United blames BankOne, and neither one is typically willing to do anything. It struck me as odd that suddenly they'd have this kind of coordination between them.