I'm not sure how I missed this before. But we spoke before about EA's thoughts on Steam sales. Valve's Jason Holtman, business development chief, responded back with a wealth of information on why they do what they do and how it works for them. But the best quote that I found from the interview sums up Valves outlook and reasoning.
If we thought having a 75 per cent sale on Portal 2 would cheapen Portal 2, we wouldn't do it.