Freedom of speech applies to everyone including athletes and celebrities. No one person's statements or beliefs divides a nation. Most people have already "picked a side" on most issues.
When a celebrity speaks out it shines a light on the issues itself for many who may have been unaware of it.
A lot of people were unaware of the troubles in Darfur before George Clooney started talking about it.
Having said that there are many people who want athletes, actors, singers, and other performers to shut up and simply be highly paid "court jesters" who entertain us. Maybe these people see such people with fame as having an "unfair" advantage to sway public opinion.
Personally speaking I have never bought any product because a celebrity advertised it and I will not form my opinions based upon theirs either. They're entitled to talk about whatever is on their minds.