This is the same man that at one point tried to say that he was the founder of Tesla.
Saying things that I agree with doesn't mean that I should inherently be in favor of said person doing X, Y, or Z.
Just look at the conditions in any society without free speech and you'll find your answer.
More abstractly, free speech is required for free thought. Without free thought, we're all thought prisoners to whomever controls the information we received. Musk's contention with Twitter is that they have been positioning themselves as that entity.