Sadly bowden it seems to be the trend from a fair few isps now days, to run their links hot :(meaning either to have them at saturation or just below saturation), as accountants tend to see unutilised capacity as poor use of money.
The best solution is yes that isp's avoid congestion in the first place by having sufficiant capacity, but some isp's simply refuse to do this so then the question is which is better, unlimited bandwidth with congestion, FUP's limits on type of traffic etc. e.g. the Three network where one cannot tether, they throttle certian protocols and various areas are heavily congested including my area, or tight usage limits but very good speeds, with no usage type restrictions, the only restriction been the amount of traffic. e.g. EE. I prefer EE's system hence I am with them.