From TCL's POV, if they give you a modem it's much easier to support.
When you consider all the brands out there now, how does the sales person know what you've actually got.
Modem would be costing TCL ~$65. If they spend 30 minutes trying to help you sort out setting up your old modem then they may as well have just given you a new one.
It's also about OOBE and having something very tangible.