Maybe she just bought into all the lies. Unless she went across the Colorado for herself.

How is it a lie? Take for example the Merchant in the Legion camp, he was attacked by Raiders in Arizona, the Legion by default will look for those Raiders and string them up on a cross, kill them, or do something to them that would make them regret that made them Raiders. If the report was false to begin with, that fate would go to the merchant for making that false report because of the lack of tolerance for breaking laws and causing trouble in Legion lands. Cass would have plenty of fellow caravan stories from honest sources that would tell her the truth, so that only would back up what was being said.

Why would Legion string up a guy they pay to lie for them? Its called propaganda. Legion want people to think he is in total control of his lands so he gets people to spread the word that is is. Just like we are asked to spread the word to NCR that Legion destroyed Nipton.

Well unless you're in their territory I don't think many people would be fond with the leigion ivading and making half of them slaves, but I wouldve liked to see more characters like the guy in megaton who worships the Enclave, and more CL side quests.