Actually you are buying it in Europe... The European Court of Justice ruled on July 3, 2012, that it is indeed permissible to resell software licenses even if the digital good has been downloaded directly from the Internet, and that the first-sale doctrine applied whenever software was originally sold to a customer for an unlimited amount of time, as such sale involves a transfer of ownership, thus prohibiting any software maker from preventing the resale of their software by any of their legitimate owners.

When the Doom source was released people used it to create a new update to date engine that required the original assets to play. This generates increased sales in the original.

steam....