Gee, I wonder why that is?
Sounds like a lot of bunk to me.
Heh, not necessarily restricted to liberal students, ya know. 
If the college in question is anything like mine was, they don’t even need volunteers. Part of your Intro to Psych class’ grade depends on being a subject for a set number of experiments. I think I had to do two. (One of them was like this one, but involved pictures instead of letters.) I’m sure the higher level courses in Psych require even more involvement.
I am not bothered that some initial study employed college kids.
However, in this particular case, we have a situation in which they correlated a political perspective to a manner in which the brain appears to operate. Since we have a lot of different indications that political viewpoints can change over time, I would just suggest that any valid serious study would include testing people whose opinions had changed to see whether the apparent brain function follows those chamges, or whether we have merely determined that attitudes of barely post-pubescent people are initially shaped in some unclear manner by brain mechanisms that may or may not carry over to later life.
Ideally, this would be done by following several cohorts in multiple sociological populations over multiple years. Less ideally, but perhaps workable, would be to gather a very wide base of people and include self-reporting of prior political attitudes in their demographic descriptions.