> but my name isn’t on those listings.

But your address is, and you also say this:

> Today I got a letter from my current ISP, Comcast

Which means Comcast has your current address. It is not hard to look up, in their own database, a name when given an address from a rental listing website.

> Also, why would they want to do this?

Their marketing department likely saw it as a "customer retention" activity. By offering to make the transfer of your Comcast account to your new location easy and effortless (they hope) their belief would be that they might retain you as a customer.

It is unlikely they were even aware of the nefarious seeming aspects of the practice.