Illegal in that case means on the side of the money. Professors, students, academics and man on the street are all in favor of sci-hub. Afterall, the totality of the content Elsevier and co. lock behind paywall or really really expensive subscriptions is provided for free by all the phd student and academics !!
Imagine working your *** off for 5 years or more then having people in suits who did nothing more than indexing it, charching insane amount to access your work. Furthermore the one being charged those insane fees are other researchers like you !