Artists should support free speech even when its their speech that is being commented upon.
That's not really how art or artists work... Good art is nearly always the product of a strict hierarchical or one-man's-vision approach to a creation. And OpenTTD is a bit more than just 'inspiration' isn't it... the interface and graphics and copied *exactly*, with extra features. It might be entirely legal, but we're not jumping off from inspiration to new game, we're copying the first game and extending it.
Also, it's not a matter of free-speech, he's not tried to shut the project down, he just doesn't like it. That is entirely his prerogative.