Data leaving their network is going to be more expensive for them.

I also have cable through them and got rid of all of our cable boxes. This was because they noticed I wasn't using them and they contacted me about that. They have a really simple way to return hardware now - https://xfinity.com/returns

Now I just stream my cable to my TVs via the Roku app (my TVs are mainly TCLs, which have Roku built in) and that isn't counted against my cap. As a customer of theirs that has saved me ~$60 a month and my TV usage doesn't get counted against my data cap. That $60 basically covers the cost of uncapped service. This seems to be a pretty good deal to me.