You're right, they should be more transparent in their advertising, so should airlines telling you that you only have a x% chance of getting there on time because of seats being oversold.

Though I'm guessing that if they didn't overcommit the bandwidth, your Internet costs would be much higher and the network utilization abysmally low. Network usage tends to be lumpy.

Australia’s advertising regulations now require advertising typical evening speeds. There are a handful of good ISPs that will (voluntarily) publicly publish backbone utilisation metrics so you can get a feel for contention. Australia’s Internet policy is far from perfect, but, again, Americans need only look basically anywhere else on Earth to see how things could be, instead of sitting around hypothesising.