GameFront has a statement from Electronic Arts saying they did not pull Crysis 2 from Steam (thanks Computer and Video Games), in spite of the implications of the "only on Origin" button they display for the first-person shooter sequel. Rather this seems possibly connected to the game's ability to sell content from within itself. Here's word from EA:

It’s unfortunate that Steam has removed Crysis 2 from their service. This was not an EA decision or the result of any action by EA.

Steam has imposed a set of business terms for developers hoping to sell content on that service – many of which are not imposed by other online game services. Unfortunately, Crytek has an agreement with another download service which violates the new rules from Steam and resulted in its expulsion of Crysis 2 from Steam.

Crysis 2 continues to be available on several other download services including GameStop, Amazon, Origin.com and more.