Keivah

Question

It seems that Keivah (rennet) itself is either waste (pirsha - Rambam, Rif, Shulkan Arukh) or dairy (Rashi), or both (R"T and Rema). Why then should there ever be a question about "non-kosher" cheese that has anything to do with maachalot assurot? Am I correct that it is entirely an issue of the gezera - which is simply not impacted by ingredients?