The problem isn't that they express their opinion. The problem is that that opinion is given a weight it doesn't deserve. When was the last time YOU were asked to testify before a Congressional subcommittee? Why not? Why is it assumed that someone famous knows any more about a given subject than some Average Joe?
THAT'S the flaw in that argument. And it's disingenuous for the "celebs" to insist that they're just expressing their opinion. They're using their status to FORM opinion, and often based on nothing more than a yen for reflected glory.