All ISPs, as a rule, oversell. They oversell at a ratio of usage in part due to how networks are structured (8-10 times bottleneck bandwidth is seen as "good" service, source: I am in the industry). There are bottlenecks in almost every network. The problem is when they oversell at an unacceptable limit for regular usage. Bottlenecks can be mitigated but that costs money and like almost every rent seeking business there is no incentive to spend any money or invest until there is risk of a losing enough users to be unprofitable.
The economics of ISPs suck which means ISPs will suck pretty much as a rule.
The economics of ISPs suck which means ISPs will suck pretty much as a rule.