From an ISP perspective having your wifi credentials saves them a ton of money in support. A lot of people do call in for that. If Verizon didn't have access to it, they then get into lengthy support calls (directing cu to get by going through router admin or worst yet a factory reset and then having to reconfigure and reconnect devices).
I do use my own router, but in the grand scheme of things I don't have an issue with Verizon having my wifi password. Unless they are within 60 ft of my router it won't do them any good. In terms of it getting stolen off a Verizon server by a hacker, all I can say is again that hacker would then have to get close enough to my router to use it, so they would have to also steal my home address. If hacker gets my home address and other personal data off of a Verizon server, I am going to be worried about a lot more than my wifii password.