ISPs lease you a line, sometimes private, sometimes shared. Most businesses, especially those that do video streaming, etc. get a private (non-shared line) and in their contract with the ISPs their bandwidth is guaranteed. Most consumers have shared lines but their bandwidth isn't guaranteed, it just usually is available because most subscribers don't use up their available bandwidth on their leased lines. ISPs sell a company or private individual a line with x available bandwidth of x' available throughput on the line. The line will always have the same capacity but because they oversubscribe they can get away with selling 20 4mbps/384kbps lines per say a DS3 (so near double the throughput of the line itself).
If everyone used the their connections the downstream would be completely saturated but the upstream would still be ok, and then consumers would be in outrage over not getting their money's worth. But consumers are already not getting their money's worth because most of them don't use the entirety of their line, and on top of that are getting nickle and dimed for every bit over X bytes/month on an already overpriced 'net connection.
This is roughly equivalent to a transportation service selling 150-200 seats on a 75 seat vehicle and then expecting the people who frequently use their service to pay more and/or turning away everyone that doesn't get on the vehicle. Either way the company is at fault and they need to pay for their mistakes, not their customers.