I'll say it : The DMZ was a naive treaty driven by goals and ideals, rather than the reality of dealing with the Cardassians. Nechayev built their policy not around reality, but around making her look good. Why else were the gum n' hairpins Maquis treated as the worstest threat ever? They weren't like the militia groups here in the US, actually seeking the overthrow of the government. I'm as far from being a Reagan-booster as you get, and yet I will say : Trust But Verify. The UFP never did with the Cardassians and the DMZ treaty, and the results were the defections of otherwise fiercely loyal officers. Not to mention those who stayed but shook their heads at it. Is it any wonder that, in Sisko's Vorta-directed fantasy, it was Nechayev who cheerfully signed away Bajor?
As to Ro herself, she did betray them. That is a fact. But she tried to not take the mission, and tried like hell to back out when she felt she would blow it. Picard worried about tea and pastries for Nechayev, and threatened Ro with arrest when she needed him. He and Starfleet were still betrayed, and if she lived, Ro should expect arrest. But no one had better dare be shocked that it happened. This wasn't Wes at the Kolvoord Hearing, and Picard insisting his way to victory bit back at him this time.