There has been some propaganda about EA removing games from Steam due to conflict in the policies. Valve’s TOS is believed to be one of the reasons behind the issue. Anyways, EA may not want its games on Steam but Valve games are available through Origin – Left 4 Dead, Left 4 Dead 2 and Counterstrike: Source. There is one restriction though, the games are boxed i.e they will be delivered to a particular address through standard procedure.
These games are actually part of Origin’s ‘buy two get one free’ offer. Which means that if you buy two games, you will get one of these three games for free. It’s not that surprising as EA has been the publisher of some of Valve’s big titles like Portal 2, Left 4 Dead and The Orange Box. So I would say that it’s a smart strategic move from EA rather than a conspiracy.