Let me play devil's advocate here and note that twitter has been used as a communication platform by what the US Gov't consider (again, I take no position on the merits) terrorist organizations,[1] and that monitoring such a public forum is at least somewhat legitimate.
The offending party probably shouldn't have been deported, but that's another matter.
[1]: http://www.huffingtonpost.com/2011/12/28/al-shabaab-twitter_...