Illegal? It would be illegal if it was free. You do pay a price for photocopying.
If you refer to the cost of the photocopying machine and the paper it is comparable to walking into a sheet music store saying:
"Come on, give me these books for free. I have already paid for the bus ticket to get here."
Would it be better to have to buy all of the sheet music for all the students who need that particular piece because of "copyright infrigement"? Of course not.
Better? The only acceptable way when using copyrighted material is to buy it, or rather to let the students by their material themselves. I believe lending is OK though.
When everyone needs the certain sheets photocopying is perfectly legal and perfecty moral to say the least.
You may of course have your own opinion about the morality aspect but it is a matter of fact that it is perfectly illegal. Read more at:
https://www.pdinfo.com/copyrt.htmBTW, has it never occured to you that if the laws for protection of intellectual properties (copyright) did not exist there would not be any music available for copying?
Henle, Wiener, Peters and the others (including Piano Street) would of course not invest thousands of working hours on researching, editing, fingering etc to create exceptional quality editions if their work would be freely available for copying and distributing.