They're not cabal members. They've only said they want to be. The only reason they can see cabal raids is as an aid; so they can choose to help and make themselves look good if they want to.

By not showing up they're punishing themselves enough already (by not exposing themselves to the rest of the cabal and forging relationships) that I don't really see a need for the cabal to punish them in other ways.

And, in the end, they're NOT members of the cabal. Fort leadership doesn't yet have power over them, except that if they want to eventually be members they have to suck up.