It doesn't violate someone's right to free speech or any other right for a company to fail to provide a service to them. A company having to provide service to some particular entity only violates one set of rights - those of the company.
It's a simple question of whether people want to support a company that acts in ways they don't approve of. A boycott is simply voting with your wallet. There's no rule that it's only right or just to boycott if laws have been broken. I'd be surprised if more than a tiny minority of boycotts were based on corporate behavior that wasn't 100% legal.
Calling it a violation of free speech waters down the impact of the phrase as it covers real governments doing it with real violence. I went to college during during the height of the political correctness bubble and was exposed to nuts who called holding a door open for a woman "rape." I couldn't imagine any more crass or insensitive action with respect to real rape victims.
But that doesn't change what they've done - it just makes you look like a lawyer making excuses for the inexcusable.
This seems to cross lines with the 'Affirmative Action'-type arguments: that in an effort to 'make up' for past wrong against black people we are making it impossible to refuse X (where X could be employment, etc) to a black person, even if the reason isn't because of their skin color.
{update} I was only attempting to clarify what I thought that the parent poster (gp to this post) was getting at.