Because knowing the difference between fiction and reality aka proship doesn't absolve you of having biases.

That said, Margo was QUICK to disobey Miguel and go against his principles when she understood Miguel was on some bullshit and was a danger to Miles. Gwendolyn Maxine Stacy did NOT. Rather, she lied to Miles the whole time either directly or by omission which put Miles in danger all the way through and even stuck to that "we're supposed to be the good guys" shit. Which makes her a perfect product of Miguel's teachings. Unlike Margo. But to know and analyze these things, one would have to not only overcome personal biases, but also know how to handle media with a critical eye AS IT IS, without overlooking the ugly parts or straight up denying their existence. Not everyone can do that. Even people who claim to the proship title.