I'm surprised that the authorities are using their access to her facebook messages for something so trivial and this openly (if they actually did). Additionally, she was not even an american citizen or resident.
In all probability, it's /because/ she's not an american citizen or resident. The fourth amendment doesn't apply for non-persons.
http://big.assets.huffingtonpost.com/DHS_CustomsImmigration_...
http://www.avvo.com/legal-answers/does-uscis-check-the-faceb...