The prequel comic (Alliance) show most of the Decepticons who came to Earth after the first movie, were killing a lot of people and causing a lot of destruction. It's only natural that those offenders needed to be neutralised, and strategically it is better to do it while they are inactive, by way of a surprise attack.
But I think it would have been better if the script had even mentioned that these Decepticons had been recently responsible for deaths and destruction, and they were moving in to neutralise the threat. But no... Bay just wants all characters to go in with all guns blazing, with no concern about character consistency...
