Clearly defining the point at which the line is crossed hamstrings them when unforseen situations arise. Not clearly defining it allows for capricious enforcement. At either end of the spectrum one party loses out. This middle ground allows for both parties to be happy much of the time, but that's not to say no improvements can be made.
I would hope serious proposals for change would take both parties into account and try to find a better compromise.
Why? Really why? I'm buying a certain number of bits per second. They don't need to know nor should care about the content of those bits.
If they oversubscribe and sell bandwidth "bursting" up to X, but only a small fraction of X for a sustained period then they should have to advertise that clearly and not claim "X bandwidth connection". Rather than burying it in a TOS.
Note that by previous internet usage patterns the sustained usage of an shared upstream connection wouldn't necessarily result in less than maximum speed if provisioned correctly based on usage levels. That may be different now with more video streaming, but only in scale, not in principle.
You are paying for a given usage profile, and the economics are based around that usage profile. A few leaches can exist for a given economic expectation, but beyond that everyone suffers.
Leaches, wtf?
Should the power company be able too ban you from using the electricity they sell you to power a saw? If you do, are you then leeching from the other electricity customers?
Just because you purchased 800 square feet (or 1Gbps), doesn't mean you can do whatever you want with it.
Any legal device connecting to any legal service so long as you don't harm the network.