This doesn't have anything to do with free speech -- they're not arresting people for posting discussion on Twitter, etc. -- but rather with reminding people to think about the consequences of their actions.

If you publicly associate your identity with anything socially or politically sensitive, you are potentially modifying your job prospects, including (in this case) potential jobs with the federal government. Freedom of speech doesn't mean you can say whatever you want with no consequences -- there are always social consequences to taking a position on something. (That's part of human nature, and you'd sooner be able to legislate love than to change that.) What this email is basically doing is reminding students that if they post about these kind of things in a way that is google-able, then they'd probably look at you the same way as someone going into a job interview with a shirt that reads "I want to steal your secrets!". (Or, imagine going into a job interview with Shell or BP wearing a Greenpeace t-shirt, for example...) Of course, you are legally free to say that, but good luck getting them to hire you afterward!

And if you never want to work for the Government in any way, shape or form, and in fact feel a desire to tell the likes of Senator Lieberman to go to blazes - and remind him of your First Amendment right to say so to his face?