It makes a little more sense once you look at the infrastructure that it's running on. You and ~500 of your neighbors are all on one local node that shares bandwidth between everyone. The way it works for upload is that your cable modem sends a request for an upload timeslot on a shared upload channel, then the CMTS which serves thousands upon thousands of people sends back your time slot when your modem is clear to use the upload channel. If anything goes screwy on your node such as significant interference or if your cable modem somehow hung up and was constantly sending on the upload channel outside of its time slot this means that absolutely everyone on the same node looses service. Not only can it affect a ton of customers all at once, your ISP doesn't have a good way of tracking down interference at a finer granularity than the node level which isn't a lot to go off of. They literally have to do a binary search by disconnecting sections of the local node and seeing what segment the interference is coming from. I've heard horror stories about interference coming from things like washing machines, treadmills, etc and being very intermittent and nearly impossible to find because of it. For the treadmill story, a whole node of customers would loose service for 15 minutes every couple of nights and whenever the ISP would get a technician out to start trying to isolate it, they'd already be off the treadmill and the problem would be over. Supposedly it took a while of having a tech in the area around when it normally started to track down which house it was coming from.

So yeah, it's your modem, but they won't let you use their network unless you're using one of their approved modems that they know are reliable and that they can manage updates on because just one person can unknowingly screw it over for a large chunk of people.