Okay so let’s use an example. Good Guys Fellowship has, in theory, two people with stalls. Guild Leader and Other Person. Guild Leader’s stall is the “fellowship” stall, so they’re good to hang out and do whatever. Other Person in theory says “yeah I’m separate.”
Now let’s introduce Third Party. Third Party likes to hang out in the Wilds and fish. He usually brings a bodyguard. Now let’s say that Other Person both goes to the Wilds and kills Third Party and take his stuff. And they call in backup from Good Guys Fellowship.
Is that fair to Third Party? Other Person just used the advantage of being in Good Guys to get his stuff and now they’re going to benefit from it and sell it. That’s not fair, they’re supposed to be separate from Good Guys!
As you can see, if we don’t have rules like that in place, we might as well just let Good Guys have all the stalls they want, because having Other Person have their own stall, but still benefiting from Good Guys’ resources, means there’s really no difference, and no competition.
Likewise, if Other Person heads out into the Wilds and takes 6 members of Good Guys with him, he’s going to have access to farm resources and be protected from bandits in a way that only a member of Good Guys can. Which means that Third Party is at a serious disadvantage now. He can’t really compete with Guild Leader OR Other Person because they have a monopoly on the Wilds!
This all makes perfect sense, and honestly owning a Market Stall is a totally optional part of the game anyway. You can choose to participate in your Fellowship’s stall as a group, or you can choose to go it alone, but what we don’t want is Good Guys owning the whole darn market.