I know that they're still recovering from the attacks on the Citadel by Sovereign and the Geth, but still, if it involves your species, at least try to do somethign about it.
Thing is though that the Terminus colonies are not in Alliance space - essentially, they are a sovereign nation. The Colonists targeted by the Collectors intentionally decided to leave Alliance space, thereby forfeiting any protection the Alliance owes it's citizens.
This is made abundantly clear on Horizon: The Colonists did not want to accept any help or protection from the Alliance; they want to remain in no-man's land where they knew they would be targeted by the enemy.
I suspect that this is again an American thing - you evidently think that it's alright to send the military to violate the territory of a sovereign nation if it is deemed necessary to protect American lives but everyone will automatically agree.
At the end of the day, you are asking that the Colonists have all the freedoms (e.g. to leave the Alliance and settle in contested no-man's-land) but at the same time have none of the risks or responsibilities (e.g. you want the Alliance military to protect them no matter what). Why?
Ashley and Kaidan don't really add much into trying to convince you that you were wrong by joining Cerberus.
True, that conversation was badly botched - the "I was dead and thus unable to do anything about Cerberus's mad science experiments which involved my corpse" explanation should have carried a bit more weight.
ME2 is well written to make anyone feel pro-Cerberus
Except not really - once again, Cerberus is the space!Al-Qaeda group that killed Shepard's squad on Akuze, and Shepard just takes space!Bin-Laden's word for everything after he wakes up; Shepard doesn't even call his buddies to see if TIM is, in fact, telling the truth but accepts everything.
If the game had demonstrated that Shepard has to work with Cerberus then it might have been acceptable (except of course for those who want to roleplay Shepard as having some kind of morals and who wouldn't join terrorist organisations at the drop of a hat) but that's not what happened...
Much like ME3's ending, you have a conversation that ends in an impasse that is then arbitrarily resolved by Shepard/Catalyst giving in for no reason at all