I agree, if the ISP purposefully limits during times where not doing so would not harm network "stability" then the ISP is in the wrong. If it is limiting people at peak hours to ensure everyone can get some access then fine. I guess my point was if they oversell enough, those hours of congestion could easily become 90% of the day.
I would rather see the system set up in reverse. A customer buys a minimum speed, and can sometimes get more than that, but the network is sized such that everyone using their connection at the same time will mean everyone gets the minimum speed they paid for. That would mean smaller numbers on the paper, and likely less money for the ISP, but I can dream. Granted I have yet to notice a time where my 20mbps DSL didn't provide 20mbps of through put.