Even when you "bring your own modem" ISPs tend to demand exclusive control while it's in service. It really is intended to be part of the ISP's network rather than yours, so while (unlike a rental unit) you could walk away with it and take it to another provider, sell it, run your own copper infrastructure, etc. you usually still can't modify the settings of existing connection.

One of my cable installers told me that rate limiting is done in the cable modem, so people would run pirate firmware that eliminated the artifical limits and run at the natural limit of the connection. People had fun with this for a while until the network engineers figured it out, and now people exceeding the speed limit get their connections shut down pretty quickly. But anyway, it makes sense the the cable modem really isn't the customer's to control.

I own and control my own wireless router because I want to play with things like DD-WRT, use OpenDNS, etc., but I see the cable modem as no different from the utility box down the street.