Coinbase has nothing to do with this. It's Coinlab who will be servicing MtGox's US and Canadian customers.
It's also MtGox who has decided they don't want to deal with the crap associated with USD transactions - they want out of the US market, so this is just part of that process. I suspect people would be whining even more if MtGox closed off their services to US customers entirely rather than transferring those customers to an intermediary. At least this way they can insulate MtGox itself and its non-US customers from the risks associated with servicing the US market while still allowing US customers to trade on MtGox. It would have been far, far worse for US customers had they just withdrawn from the market entirely.