It's an issue of control:

"Think for yourselves and let others enjoy the privilege to do so too." (From Voltaire's "Essay On Tolerance")

"I detest what you write, but I would give my life to make it possible for you to continue to write." (From a letter from Voltaire)

"I disapprove of what you say, but I will defend to the death your right to say it." (First used in quoting a letter from Voltaire to Helvetius, in "The Friends of Voltaire.")

All the above citations come from Bartlett's Familiar Quotations.