If the user does not own it, someone does and the accounting should play out accordingly.
I remember reading that, back in the 50s or 60s, the phone company owned "your" phone. It was permanently attached to the wall, and you weren't allowed to do anything to alter it. Did AT&T pay taxes on those phones as inventory?
Wish I knew about inventory. My guess is yes! They would have to do accounting on all the modems anyway. Service, where they are all at, serial numbers and more.
When one went bad, it usually involved either a visit to the ISP, or a tech shows up with a new one and a few records get updated and the modem ends up home.
Cable TV boxes are another example!
I do know those were inventoried. A friend went to work as a tech for a local cable company. (Yes, they did tell me how to enable decryption on all channels for at least one model...) They fixed the incoming boxes and those units went right back out to homes.
The units were purchased a few times per year to balance subscriber growth and attrition. Good repair metrics saved a TON of money. The units were from one to a few hundred dollars a pop!
Rental was $7.99 / month at one point I could remember.
Say the box was $150, at that rental rate the box becomes a minor profit item after a year and a half, right?
Well, most cable boxes got used a decade or more, especially when the company did not have to change its encryption tech.
That is a 4 to 5x return on the purchase price. Maybe 3x return after attrition, failed returns and repair costs get woven in.
Not a bad deal for them. And as the user, not many worries, but also basically zero options.
An example from cable boxes was serial ports and input and output ports either disabled or nerfed to the point of poor usability really sucked for anyone wanting to use the gear technically, or as part of an automation of some kind.