

The US originally also charged per minute for dial-up connections. Dial-up after all was making phone calls (even on ISDN).
They did get flat fee models while still in dial-up probably due to earlier adoption thus earlier competition than in EU countries.






The term customer is used in regards to the ISP who are supposed to track traffic. For the servers it refers to users, not customers. There does not seem to be any differentiation with regards to the type of data, so yes even CSS or plain text would count (though they obviously would be too small of a quantity to matter).