Because it has been shown that even if freedom is speech is a right that people will still be afraid to speak up if they know they are being monitored. Because the reality is that even if you are allowed to speak you might still be punished through other means.

This is much bigger than saying something illegal on the internet. This is about not being able to criticize your government without fear of retribution. Or how about if this was possible 60 years ago. Gays would have been all caught and gay rights never emerged. Or say you are discussing wanting an abortion and are arrested for arrested for it because at that point in time it is made illegal. The right to have private communication is integral to a free and democratic society. Morals change. Beliefs change. This is good. If you are monitored for everything we will be oppressed and stuck with no way to progress and grow.