I've had some time to think this over and I think Optimus's reaction was a bit tame when you consider the fact that someone could very easily have died (and I don't just mean Starscream either).

I'm also surprised that he didn't say anything to Starscream. Strategically-speaking having the 2IC of the whole frickin' enemy army (apparently) defect is a major coup. The information Starscream has about the Nemesis, about the Decepticon Army back on Cybertron, and about Megatron himself, have the potential to end a war that's been going on for a gazillion years. In light of that, it's in Optimus's best interests to keep Starscream alive and (relatively) sweet.

Of course Optimus probably isn't as cynical as I am. ;)