New modem lease cancel

When we first joined Cox last year we bundled our services and ended up leasing a panoramic wifi modem (EMT) for our internet and chose one of their home telephone packages. Fast forward to today and we have since canceled our home  home telephone service. We never used our home phone so we canceled and decided to get a better modem rather than our Cox modem. When I went to install our new modem, I called Cox customer service to cancel our leased modem and register our new modem to use. During our installation I was told Cox could NOT activate our new modem until our leased modem was turned in and inventoried at a Cox store. This makes NO SENSE to me. I fully intend to return leased equipment and even if I didn't I understand that I would be charged full price for the cost of the modem under my account. Generally, I've seen most carriers give you a deadline, maybe a week, to return leased equipment but still allow for new equipment to be activated. Additionally, I was told because I had a certain leased EMT modem that Was in my house it HAD to be returned before I could activate my purchased modem. Meaning, if I had a different modem I could have had Cox remotely activate my new modem. Can anyone explain this to me as why this is the case?