Because they also sell a phone.
But here we have venteprivee.com where like 3 or 4 times a year, operators put an offer for a lower price for their service. Every year, if you look around summer holidays, you can get internet for 2€/month instead of 20€/month and sometimes phone plans too for a very low price.
It usually only last for a year and then you go back to normal price. If you are like me and you don't care about keeping your phone number, then you can just cancel your plan whenever there is a better plan or at the end of the offer :)
It's funny because every operator uses that english term