You can make this argument at various levels:
"Speech may influence someone to act in a certain manner, but it doesn't cause them to act that way—they have to choose to act, and in doing so take full responsibility for the consequences."
Replace "speech" with "bribery" or a mafia boss giving an order to commit a crime.
Causality, in the sense that you're using it, is legalistic. In practice, the Jonestown religion/cult, it's beliefs, and membership did cause a mass suicide and murder. That obviously doesn't absolve anyone of anything, but you can't exonerate the religion on a philosophical technicality. Religion is a right. That does not mean that it is harmless, powerless or unrelated to causality.
Speech is just the same. Voting too. Powerful. Dangerous.
If you believe that free speech needs to be protected because it causes no harm... that's the road to losing it, because it's premised on a demonstrable untruth.