Are professors at a public university in the U.S. considered to be "employed by the U.S. government"?
The reason this question is asked by journals is that except in some limited cases, the US government may not, by law, hold the US copyright in a work created by its employees, and, therefore, cannot transfer it to the publisher of the journal as is traditionally done in most academic publishing agreements. As such, the journal can't ask those authors who are US government employees to sign the copyright transfer and must use a different agreement. If one or more of the authors is not a US government employee, then those authors will still need to make the transfer. 

Employees of US state governments, which includes most public university employees in the US, do not have this limitation, and can make the copyright transfer or other licensing arrangement that the publisher requires.