No doubt you're right that one could sue them in theory. But it would not be worth it for $400. What remains is to let others know about their hefty withdrawal fees and strange customer service attitude. [I use the term "strange" because it is hard to understand why Bitstamp would be trying to prevent investigation of theft of customer funds by banks.]
Update: Their manager did offer me a month-long 10% rebate off of trading fees to compensate for this. That was a nice gesture, but kind of meaningless, since I would have to buy/sell almost 6000 bitcoins to recoup the loss.
If you're UK resident then you could just use the Small Claims process to get the overcharged amount back. It's not that expensive, and if you win, you can get all the costs back: fees, interest, or even cost of your time you've spent on making the claim.
Not sure is that an option for non UK residents, but you can still write them a formal letter requesting either money back or proper breakdown of the fees they charged. If they don't reply, you can make a complain:
https://www.gov.uk/complain-about-a-limited-companyhttp://www.which.co.uk/consumer-rights/action/how-to-complain-to-a-company