I think Peterson is concerned that C-16 might reduce our Freedom of Speech. At its basics, Freedom of Speech means you can say whatever you want whenever you want. You lose some of that freedom when you cannot say certain things, but forcing people to say certain things is on a whole other level. The best way of thinking about it is the Newspeak in 1984. The government removed many words from the people's vocabulary and only left words that align people's speech with the government ideology. From Sparknotes.com: "Newspeak is engineered to remove even the possibility of rebellious thoughts". If we were forced to call them irregular immigrants, over time, we might not think of it as illegal anymore. Firstly, if no one is allowed to say "Illegal", we lose that concept. Secondly, if we are forced to call them irregular immigrants, the government is dictating how we should think.