Word for someone using their fame to push their personal agenda

Ed may be a promoter of the idea. He may also be authoritative on the subject. Neither of these says he is not correct or excessive in his views.

As in:

Ed (a famous sportsman) is as a dilettante, using only his fame to promote political views.

or

Ed (a famous sportsman) is being dilettantish (showing frivolous or superficial interest; amateurish), using his fame to promote his political views

dilettante, in the following sense: TFD

1. a person whose interest in a subject is superficial rather than professional

The common phrase I've seen for this is "celebrity activist"--see news examples here at Huffington Post. Google searches also return this phrase in tabloids and pop culture magazines.