A smaller ISP near me (Bright House) does a similar thing with their hotspots. There are no ad injections, but they inject a little popup with their logo, that says something along the lines of "hotspot provided by Bright House networks".
The funny thing is, this network requires you to log in with your Bright House username and password, so anyone seeing that little "provided by" intrusion is already a paying customer.
The funny thing is, this network requires you to log in with your Bright House username and password, so anyone seeing that little "provided by" intrusion is already a paying customer.