Cut block is not illegal. You are thinking of a chop block.
"...cut blocking is an offensive line technique that consists of an offensive player knocking a defensive player down by hitting his knees. The technique, which was initially instilled by Bobb McKittrick, the offensive line coach of the San Francisco 49ers from 1979 to 1999, is often criticized as being "dirty." Additionally, it is often illegal for an offensive player to "cut" a defensive player already engaged with another offensive player. This is considered a "chop block." In the NCAA, cut blocking is allowed as long as the block is no more than five yards away from the line of scrimmage and it is inside the tackle box."
While some still thinks it's dirty it's totally legal. I figured you misunderstood. Your linemen use this technique because it can nullify bigger faster D linemen if done right.
I also hate playing teams who do it because it increases the chances of injuries and thus why the NFL banned it. They banned it this year because it's highly controversial. And leading up to this years ban in the NFL many saw the technique as unethical.
Cut-blocking is legal, but is it ethical? - Houston Chronicle
So while your coaches don't teach an illegal move by your line it might be seen as unethical, there are coaches out there who refuse to teach it. I know I would.