There is a world somewhere between reality and fiction. Although ignored by many, it is very real and so are those living in it. This forum is about the natural world. Here, wild animals will be heard and respected. The forum offers a glimpse into an unknown world as well as a room with a view on the present and the future. Anyone able to speak on behalf of those living in the emerald forest and the deep blue sea is invited to join.
I guess it is comibination of these:
a) they can't chase him away and with his current size it is not worth the risk to challenge him at all cost unless it becomes necessary, which they clearly don't consider be the case
b) as long as he is not behaving like a threat for cubs (Tintswalos don't mind the latest Mbiri cubs in their natal pride as well, ofc it helps if he male(s) are related to the mothers) and is not challenging them for mating rights, he is essentially just olstrign their numbers for food
c) that is likely to change eventually, but even then, it could give them more longevity as dominant pride males, to some extent regardless of their condition, no matter the assumed coalition dynamics would change in BYM favour over time (the blond mane male is already clearly second fiddle most of the time, even now)
d) more importantly this gives current litters much better chance to make it, security wise, no matter how Ross male would age well in next years, and perhaps few more litters after these (I don't mean few more round of littters)
e) this all seems nice to have but not that needed right now, as there doesn't seem to be strong challenge for either of Birminghams prides, but that might change anytime and maybe they are aware of Mbiris - in this case getting a third male, rather than risking injury by getting rid of him, seem beneficial; at the same time they might still have Birmingham breakaway pride to fall into if the original Birmingham pride gets taken or vice versa
f) for the sake of mentioning it, while I don't think that is something they might instinctively consider, BYM's limp might eventually work in their favour if he can't run that fast, either by not being able to chase either of them properly if the relationship with him go south on day or in case of suffering a succesful takeover or even just losing a fight against a stronger opposition, BYM is more likely the one not to keep up, ending up left behind nad being caught ; but chances are before anything like that happens Ross males might acquire injuries on their own.
Still that doesn't explain anything, just factors that might influences it, the thing that is a known individual that the pride don't mind around like an alien male would is likely the most decisive one. It would be interesting if this would affect kicking out of the white male and rest of the sub-adults.