I think in many first-world nations, distributing info intended to enable others to commit a crime is, itself, a crime. (Which is not to suggest I agree with the state's legal ruling here.)

Edit: but of course the referendum itself being illegal does not necessarily imply that voting in it, is illegal. And it seems more reasonable that the state could determine the results of the referendum to be non-binding per their constitution, rather than being justified in preventing it from taking place.