What a stupid statement. Are we to believe that everyone who graduates from college with a bachelor's degree has somehow been brainwashed into a liberal?
Brainwashing is not a guaranteed effect. If you attempt to brainwash everyone, some may be brainwashed while others will see through your attempt, or simply be unaffected. Therefor, if we agree that American universities attempt to brainwash students into liberal politics, then a group which is more exposed will likely be more greatly affected.
Not that I agree with the entire concept, but it does make logical sense. I think the more plausible culprit is the fly on crap analogy. That is, liberals want to be journalists because they believe it is a sacred profession. One would expect pious persons to enter the church, and likewise one would expect liberals to enter journalism.