Well it probably was, among other things. But we're censoring a network here, not just a tool. Nobody is making the cryptography behind Tornado illegal.
This is known as “chilling effect” in a legal context.
People who might otherwise want to use this for financial privacy would be wary, since if they deposit funds to the contract, they don't have any way to know if they'll be able to use the unlinked funds later if withdrawn.