While the initial statement may see crass, there maybe some merit in the thought behind it.
It's not cool to be seen as following the same mantra as those in power, with the tory government in place it wouldn't be right for Eddie Izzard and that coterie to stand up and say that Teresa is doing a great job.
The arty types, musicians in particular have the ability to draw people in and modify general cultural thinking; its easy for politicians to garner their support when they need it. Similarly with the LGBT groups, it's not "cool" to be seen as a right winger therefore the perception is that that they are all left wingers when that's not necessarily the case.
There also some truth to the fiscal comment made early in this thread, "most" of these "arty types" under discussion seem well off and slot neatly into that wealthy, elitist, left wing, liberalist stereotype, rightly or wrongly; perception is all and sometimes we are guilty of percieving reality!!