An analogy is the government forcing you to draw something objectionable, and then sign your name to it. That's clearly unconstitutional compelled speech, plus the mere existence of an obscene work signed by you could seriously harm your reputation as an artist.
Here the creative work is software, and the signature is digital, but the principle is the same.
Here is how one commonly goes about defining free speech, "Freedom of speech is the right to communicate one's opinions and ideas without fear of government retaliation or censorship."
If you can compel speech, then you can effectively censor people by forcing them to disavow their messages, and then burying their signal in compelled noise.
Or to put it another way, compelling speech is denying the right to send the null message.
Here the creative work is software, and the signature is digital, but the principle is the same.