I'm pretty sure there's a law against this

a very specific one, actually. C/o the Law Librarian Blog:

DOD Spying on Student Protest Groups Extended to Monitoring E-Mail

The Chronicle of Higher Education is reporting that the DOD monitored e-mail messages from college student who were planning protests against the war in Iraq and against the military's "don't ask, don't tell" policy against gay and lesbian members of the armed forces, according to surveillance reports released last month per a FOIA request by the Servicemembers Legal Defense Network. The FOIA request covered government surveillance at the State University of New York at Albany, Southern Connecticut State University, the University of California at Berkeley, and William Paterson University of New Jersey. [JH]