Should he be angry? Surprised? A loophole isn't going to help with the whole 'trustworthy' issue, now is it? Bumblebee isn't really surprised, not with the flat look he gives Megatron upon hearing that. It's to be expected and, in the end, can he really blame him?

"I bet Optimus was actually relieved that you did. Though I guess being on the Lost Light is punishment enough."