Two reasons that wouldn't fly. The available server(s) would likely be overloaded into uselessness with no purchasing barrier in place and I doubt Valve would be willing to foot the bandwidth bill for those free client distros w/o receiving compensation.
Well Valve allows free Demo downloads, and some of them are quite large. Not to mention people are downloading free 2 play games off of Steam now. And bandwidth? Surely it wouldn't be any worse than on Steam sale days when Steam just quits working for stretches of time because it's massively overloaded.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi