It's different because the law Brian is charged under prohibits unauthorized access. There isn't a law that prohibits unauthorized access to cars.
I think the breadth of 18 U.S.C. 1029 (unauthorized use of an access device) is really illustrated here. NPPD said they did not think they could charge Brian with theft under Florida law because there is evidence that he was authorized to drive the car and it was "commonly controlled." That is his interpretation or the interpretation of the NPPD lawyers, not mine.
The feds had no problem charging him under 1029 because the "access" comes from the bank. The bank is a third party that can say the access was unauthorized because the Card and PIN were not issued to Brian.
In my personal completely biased opinion, I think the feds ARE properly using the unauthorized access law (1029) but it shows why the statute is overly broad and bad IMO. You can charge under it when you couldn't charge theft. JMO