At least in my country, Germany, fraudulent use of telecommunications services, public transport or paid events/facilities ("Erschleichen von Leistungen", § 265a StGB) is not just a civil matter, but also a criminal offence. If you got caught manipulating your modem to avoid paying for faster broadband, you would likely to get into trouble.
What if you used the exploit to change someone else's modem speed? If you are the who's modem was modified, how could you prove this in court? It sounds like it'd be very difficult. However, if you can somehow prove this, then you can just alter your own and claim someone else did it externally and claim it's their own fault because they provided a device that was exploitable.
Note, the premise of this comment is based off another commenter's claim that you can pull this off remotely and don't actually need to be on the local network.
Uncapping a modem to get more speed used to be possible but I can't imagine how it could be useful: It takes two to tango, so to actually go faster you'd need to get the other party to go faster as well. The other party being the ISP you're attempting to cheat, which has every incentive to monitor such things and act on those monitors.
Morality aside, I just don't see how it could ever end well for the uncapper.
Most US cable providers let you use your own DOCSIS modem. I doubt there are any legal consequences for changing config on your own modem. Channel bonding has to be configured on both sides though.
Eh, people have been raided and charged for the distribution modems and running websites. But cases of theft of service for running a hacked modem are nearly non-existent.
