That is a bit misleading. Kolakowski was the leading Party intellectual from 1947 but then fell into disfavor and left the country after the 1968 student rebellion. That may explain his anti-communist position as an émigré - vast majority of Eastern European émigrés became rabid anti-communists.
My explanation of this pattern is pretty much the same as I proposed for the political attitudes of IT workers a few days ago - downward mobility produces fascist attitudes. Most of these EE intellectuals were virtually gods during their heyday, thanks to Party patronage of course, so falling into disfavor must have been really hard on them. But unlike Boetius who shared the same fate and looked for consolation in philosophy, these guys used the skill of sycophancy they learned in EE and tried to regain their privileged position by providing the needed conclusions to their new masters.
Prior to his exile, Kolakowski was a good, thoughtful writer, but after that his rabidly anti-communist rants were unreadable. I think he is somewhat like Heidegger - a few good ideas but a political sell-out.
PS. An excellently sarcastic portrait of the EE intelligentsia can be found in the novel _The Joke_ by Milan Kundera.
Wojtek