My question was more:
If an ISP usually has enough pipe for everyone on each of their distribution nodes (that is, has no oversubscription—or, more practically, only oversubscription for aggregations of customers with statistically-predictable usage such that the node should reasonably never be saturated, with some extra engineering tolerance for the rare occasions usage goes above statistical norms)—
but the ISP sometimes falls short when the introduction of a new service like VoIP or Netflix or Steam into the market, or lots of new denser construction in an area, makes demand outstrip their supply,
then should they get in trouble for enforcing QoSing (through whatever means) on their distribution nodes, temporarily, while they work on upgrading those nodes?
Because, if so, then ISPs should probably never use the word "unlimited" at all because they would still get in trouble for reasons (almost) completely out of their control.
(I say "almost" because they could just refuse to sell service in oversubscribed areas. But customers would like that even less, if the ISP has a local monopoly. Alternately, ISPs could change to "dynamic billing" where everyone in an oversubscribed area gets billed for non-unlimited service for as long as they're oversubscribed, and then goes back to being billed for unlimited service once the oversubscription is over. This would be cool, but seems like one of those nearly-impossible-with-the-hardware-we've-got sort of things, especially for wireless ISPs where you don't stay on the same distribution node so you're constantly moving between oversubscribed and undersubscribed nodes.)