Everyone "transmits money" in the course of doing business. The "customer request" here is "give me my money." Gox is not sending money to other people at customer request. The customer has an account that they are either depositing into or withdrawing from.
It's understandable that people are upset. However, there are rules. Not only are there rules, but the rules are fairly clear. Not only are they fairly clear, they were further clarified a couple months ago.
There is no excuse for MtGox to not have registered with Fincen.