Why would Calvera let them go?
Seriously, he had them at gun point. All he had to do was gun them down and bury the bodies somewhere. And if he really had to dump them outside of town, he should have had the foresight not to give them their guns back. That just felt like a bad plot device. And he didn't even bother to post any sentries to make sure they didn't decide to come back. Seriously, just ignoring the presence of 7 deadly, armed mercenaries wasn't that believable.
Also, why were Lee, Bernard, and Britt standing in the open like morons, risking their lives needlessly, while the bandits were either retreating or all dead? If they'd stayed behind cover they would have still been alive.
~ I'm a 21st century man and I don't wanna be here.