How could they? It is traffic going back and forth to the Netflix servers over https. The ISP can't tell WHAT is in that traffic, only where it is going. Thus, if they throttle Netflix, they throttle fast.com.
The simplest way would be to cheat and prioritize traffic for a few minutes and then throttle. That way, any speed test would come out fine but not actual shows.
As others have said, the simple way would be to optimize for a time period. However, another way would be to monitor the site and watch the data patterns that emerge. using those data patterns, the ISPs will be able to have a better idea of how to game the test.
