Now lets think about this for a minute. At the end of the first movie Optimus sends an invite to other Autobots to come to Earth. These Decepticons were not invited. Even if they aren't doing anything then they are still tresspassing. Maybe they need a big sign hanging from a satelitte in space "Tresspassers will be shot!"

Also it's like I've seen in numerous Law and Order episodes. Why did they run when the authorities show up? Jerry Orbach used to ask it all the time "If you didn't do anything wrong then why did you run when you saw us?"