Why cannot I use Arris TG1682G bought on eBay?

Hello,

Here is the story. I bought a new Arris TG1682G (COMCAST variant) unit in unopened original packaging on eBay. I went to the xFinity store to get my old unit removed from my name and register for a new unit.

The store customer service representative told me that by looking at the serial number on my new unit she can tell that this unit is registered for some other customer and that unit cannot be transferred to my name. Basicaslly, the representative said that the TG1682G manufactured for COMCAST cannot be used because they are "for rent only".

I don't understand that policy. I lawfully bought the unit. xFinity lawfully uses the same type of modem and still they prohibit me from using the same type of device.

The phone companies do not prohibit using phones bought from elsewhere they just require releasing IMEI number from the previous user. In this case the xFinity is the only company owning the serial number. 

Why xFinity have that policy? Or is it just a customer service representative point of view?

Is there a way to do what I planned to do?

Thanks in advance!