...nothing says you are bidding at a "rate for up to two people."
This is correct. A big x-factor with PL is you cannot specify room configuration - it can be one king, two doubles or queens, etc. All that is guaranteed is that the room will sleep two people. There is no TOS prohibition against having a third person in the room, but you might get lucky and score a room with two queens, and you might not. If the latter you have no leverage to do anything about it.
This is why PL is such a rough fit for family travel.
This also leaves aside the whole question of Spanish (and European) hotel rooms often being not configured to accommodate more than two people anyway. You get single beds a lot there which hardly ever show up in North American hotels.