Stupidity: Andor and Luthen set a vehicle to drive and distract the security team. The vehicle is destroyed, and they immediately round the corner in full view of the team on a speeder and drive away, then blow up the vehicle. They could have driven away on any number of other streets without revealing that they weren't in the vehicle, keeping their escape secret. And if they blew it up before the team looked too closely they might have even been deemed to have been killed in the crash. (00:32:19 - 00:32:43)
Answer: I agree that the aliens' motivation is not clear, and this may all be a writer's shortcut to get Andor and Melshi on to the next part of the story. But I believe they are very disgruntled with the Empire, and I suspect they may be pretending to be more hostile than they really are, just to wind Andor and Melshi up. So - with no other evidence - I think they give up their ship because it messes with the Empire, it helps some mistreated prisoners escape, and the aliens have nothing better to do than walk back to their homes.