Cable Companies and Modems

My 2 year $50 internet per month currently 800MBPs speed with Xfinity is just coming to an end so I went to look at renewal options.

Saw that new customers could get 300MBPs speed for $25 for 2 years with free modem/ router. My price would be $45 without modem / router.

So I asked them for a better renewal deal. Got the runaround so signed up in my wife's name as a new customer.

Modem / Router has a $15 delivery fee but worth it because it supports faster upload speeds which recently came to my area. So will get 300/100 for half what I was paying for 800/25.

Annoyingly, I only got my modem a year ago but hopefully in 2 years, the ones that support higher upload speeds will be at a better price.

What is interesting to me about modems is that apparently my modem actually would have worked on the new account because it is at the same address. But people who buy used modems often have trouble.

In other words it appears that cable providers lock MAC addresses to property addresses. If this is true, why haven't the FCC addressed this? A modem belongs to the person who paid for it, not the company that first activated it.

Never heard of that.
I have encountered (bought) a used modem that was blacklisted by one cable company but it worked fine on another.
I don't think they share their blacklists with each other? It was quite a few years ago.
The company that blacklisted claimed it was still attached to an account but wouldn't give me any other information (like if they considered it stolen or money was owed on the account). Seller would have refunded but I just used it for other companies, still in use to this day.

So maybe it's just on modems in accounts with balances outstanding. Some people have complained about non new modems purchased from Amazon for example.

I think it's something more like that.
I think that the more professional refurbishers have ways to check them.
It's been a while but I recall many listings on eBay would say something like "not for X service" or "only for use on X" even though the model was listed as supported by all the companies. Sometimes there were sub-model #s that were more company specific. Perhaps there were "locked" models or ones with specific software? IDK, maybe a bit akin to a carrier sold phone vs. factory unlocked of the same model?
All quite opaque to me.
I think if you buy from a solid seller you shouldn't have too much hassle returning it if you run into an issue getting a used cable modem up and running. I'm sure they are well aware and keep up on the issues vs. buyers trying to scam them.
I once had a know nothing tech support person who couldn't provision the modem and said it was bad but HUCA fixed that.

Jut to add, in your scenario where your wife is a "new" customer and if you own the modem that has been in use previously at that location.
I've told cable co that the previous resident agreed to leave the existing equipment at the residence.