No it isn't.
Its very well advertised, and very specific.
They cut you to around 60% of your maximum speed for a period of 5 hours if you go over the amount stated.
The amoun isn't small, and only downloading of large amounts of data, such as a game install will push you over it.
Steam is one of the reasons someone recently started a thread to see if he could drum up interest for a VM steam mirror, so they can cut their internation traffic but remirroring steam directly to their customers.
I had this yesterday when downloading empireTW and napoleonTW, the downloads came to 39.5GB in total, started at full speed 2.1MB/sec and kept at it until I hit my limit, then I was throttled to around 600KB/sec for 5 hours then the throttle was lifted, I returned to 2.1MB/sec until I hit the limit a second time where the speed dropped again.
I think it is fair, I still go the data in under 12 hours, beats any postage, and beats most ISPs full speed also.
If I wanted to be free of throttling on steam I could upgrade to 50Mbit service, and then only get throttled if P2P ports were in use heavily.