fair enough
that's part of their arbitrary rules that affect the player exclusively. In theory all clans should be able to own caravans and enterprises, but that never made it into the game (it was implied by the first few statements they made about how the game would work where AI and PC would follow more or less the exact same landscape - at this point I strongly believe that will never happen)
It's probably tied towards being within a faction and being within a faction makes no distinction of "mercenary" or "vassal" on many mechanics - once you are "included" you become subject to the same rules. I too would like that change but than again I don't see much point in it considering enterprises are such low value, even overtime. It would be best if your income was instead sequestrated as it used to be in Warband though...