but see, that's the problem, isn't it? is the EMS really an outside power? recall that Madara genealogically ordered the doujutsu line.
The Sharingan can decode some of the tablet, the Mangekyou even more, and then the Rinnegan can do it completely. You can see that the Mangekyou is genetically closer to the Rinnegan than the Sharingan, although all doujutsu are essentially one and the same genetically. note that Madara does not differentiate the Mangekyou from the Eternal Mangekyou, rather he only refers to one and that is the Eternal Mangekyou since it is the perfected doujutsu. in other words, the
EMS, despite transplanting another pair of eyes, is a way of partially restoring the lost genetics from the original doujutsu or at least getting as close to it as possible. it is essentially a part of Sasuke's DNA, it's in his genetics. I know this sounds weird and disgusting, but you have to remember that the Uchiha clan is cursed.
to better explain, let me give you an example. imagine that your great, great, great grandfather had telepathic powers. it is certainly a unique ability and that ability was genetically inherited by the next people in line. however, you being born into that family line, a few hundred years later, find that the telepathy genes are either dormant or are not strong enough, despite inbreeding between close/distant cousins, to prevent genetic pollution. if I were to offer you the choice of restoring that gene by using your ancestors' retrieved DNA and genetically augmenting your DNA, would you take it? let us say that you did end up taking that offer (without hesitation, if I might add) and a person comes to you and says that you are using an "outside power" or something that is not yours, now what? Do you see how unfair that sounds? you have the genes, despite being dormant or too weak for any significance, they are in your DNA and belong to you. can anyone say that it is not yours? sorry for the weird example, but I thought it was pretty creative...
anyways,
if it wasn't a part of his genetics, then any shinobi can simply transplant another pair of eyeballs to his own and he would awaken a new doujutsu, even if he doesn't have the sharingan. Neji could transplant Hinata's Byakugan, 1+1=2, and hence, he would have the Eternal Byakugan, wouldn't he? Konohamaru can transplant Asuma's eyeballs and then he will awaken the Eternal Normal Eyes, normal but special

. the only reason why the EMS exists is because the
clan members have the genetic potential to awaken the new doujutsu. the new doujutsu is in their genes, it doesn't come simply from adding eyeballs to another pair. the EMS is in Sasuke's genetics, as well as all other Uchiha clan members. the Mangekyou, both the imperfect and the perfect or MS and EMS, is a way to gain power that is as close to the original doujutsu, the Rinnegan (or "true" sharingan). it is essentially the "next best thing".