Even if they could steal your bitcoins, I doubt they're stupid enough to do that. Imagine how it will affect their business and consumer trust. Sure, they can simply brush it off by saying "we don't steal it, maybe somebody cracks it", but I don't think they'll go full Mt.Gox.
The moment it happened they would have the world jumping down their throat. I have no doubt it has piled up into a significant amount of coinage by now, but there's also the age old thing of coins not moving not automatically meaning they're 'lost'.
Some day some service somewhere will announce a policy along these lines. It'll be a right old mess no doubt.