The key here is that it doesn’t use any data from their peer connections (connections to other ISPs) which cost them money, they will likely have those movies on a server local to their network so it isn’t costing them any more money to shift those bits down the line to you.
So it actually is cheaper for them if you sit all day watching their streaming services vs the others - unless they have a “CDN” or Content Delivery Network, which is just a fancy name for “a server owned by the likes of Netflix or the company they contract that out to such as akami installed in the data centre of the ISP which will serve the video files to you from there rather than across the wider internet via a transparent proxy setup”.
In effect it actually uses up no data in the eyes of the ISP because they only care about the stuff which has to come from someone else’s network as they have to pay for that, which cuts into the profit they make from you.
