I owned my modem - why do I get charged a fee

I owned my modem - why do I get charged a fee I have used two modems (both I installed myself and setup myself.) I have never received a modem from cox but am being charged a modem rental fee. when I chatted I was told this is due to "Wifi service fee" ... clearly this can't be. my personal device provides wifi to my house and cox provides the internet... someone help please before I research ATT Uverse