This issue of entertainers and their political views is obviously weighing on a lot of minds, considering the rash of Springsteen threads. And, as I'm trying to think through the issue, the more complicated it seems.
Most people who have posted who dislike hearing entertainers political views also seem to be expressing a frustration at the lack of voice for "the rest of us," those citizens who don't have the same exposure, who have strong political convictions but don't have the same access to global communication networks. And, if this entertainer is expressing a political belief opposed to yours, it can certainly be frustrating. For example, I think Jessica Simpson is silly. I hate the tired blonde act (which she seems to admit is an act), her voice is fine but not spectacular, and the songs she sings don't seem to have any substance. Plus, I'm bombarded by her image everytime I turn on the TV, walk through an airport and see covers of magazine, etc. But, I'd be fine with her using this status to support the president (if that's her view, as another posted suggested). Heck, even though I'd likely disagree with her support, I'd be more impressed with her as a person for making a political public statement and hopefully encouraging her young fans to become active in politics.
Maybe I'm not frustrated by these public demonstrations by entertainer (on either side) is because they are just that...public. I acknowledge that their money buys them media access. But they aren't trying to hide their beliefs or, I believe, cram them down anyones throat. They may say "Vote for X," but you still have the power to cast your own ballot.
Personally, I'm more frustrated by political power that I can't see, the power that corporations have to change the landscapes in which we live, to have access to local and national lawmakers, to reek environmental damage on global proportions...and then to have the ability/money to hide those intentions behind well-placed public relations schemes. This is stuff I feel no control over, partially because the process is so hidden.