The degree to which self selection plays a role is an interesting question. Firstly, there is a certain amount of overt harassment--for example, professors in the humanities and social sciences openly admit that they would be inclined to reject a candidate if they knew he or she was conservative. Also there are lots of other data, anecdotal or quantitative. Much more is needed, however.
Importantly, the proportion of conservatives to liberals has been falling since the 90s, so something is driving the trend. It's also possible that conservatives are selecting out of a career that they (evidently correctly) perceive to be hostile to them or otherwise corrupt (more interested in furthering an agenda than seeking truth). The point is that even if it is self-selection, we may be able to improve the ratio enough to improve the science. And it's not like the ratio needs to be 50:50--probably 30:70 would suffice. Right now many fields are less than 10:90, and intuition suggests that the effect is nonlinear. If only 10% of faculty are conservative, intimidation causes them to suppress their already marginal voice, but 30% might be enough to allow them to feel secure in providing the sort of criticism those fields seem to desperately need. Of course, I'm picking on the academy, but the same probably holds true for journalism.
Again, the point isn't to assign blame, but to improve the science.