There is a point of view that says that computer science conferences exist for the purpose of gaming the tenure system. The name of the game is plausible deniability: you're not supposed to submit papers that contain known false claims, but everything else is fair game. And this has become such an integral part of the culture that technical correctness is no longer a necessary condition for accepting a paper [1]. I think in this light it's quite clear why many scientists are happy to leave their papers hidden behind the ACM paywall.

[1] http://agtb.wordpress.com/2013/04/14/should-technical-errors...