Because it let's them provide better service (as defined as what most people care about) to the majority of people. Better, in the context is cheaper and faster. Do you think they can provide the same speed for the same price without any limitations? I don't believe so, but feel free to make your case.
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.
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.