Except there is no such thing as "Skyrim logic". His logic was "because one faction does this, this other unrelated faction should also do that"
No, but they want the same rules to apply. It is not impractical for one to want it considering the games logic on the whole mess. If you do not then you don't have to desire it, but having a mod that makes you join after you free the prince in rags would make perfect sense.
They're not the "same rules". Joining the companions is not the same as helping the Forsworn. The Forsworn are violent to outsiders, companions are not. It is possible to help the companions without joining them. It is possible to help the Forsworn without joining them.
Personally I'd like to be able to side with the Foresworn. But then, I also want to 'cure' the Falmer, bring them back to the surface as Snow Elves, and kick the Nords out of Skyrim. >.>
I'm sure a lot of people would like to side-with the Forsworn, but I'm not so sure the Forsworn would like anybody to side with them. I think that's their exact purpose, remaining separate from everyone else. They'd never be friends with a Stormcloak or Imperial sympathizer, or a Nord, or a Khajit, or an Argonian, or any type of elf or anything that isn't another Forsworn.
By that logic we know, one way or the other, that it is inconsistent.
It isn't inconsistent. They're not the same group, why should they be treated the same? You cannot join the Forsworn. The Forsworn are violent to outsiders. The Forsworn have no interest in letting outsiders join. They have absolutely nothing in common with the companions. There is absolutely no reason to even compare the two groups.