Not just the US government, but anyone else. A license is essentially a contract, to be effective, it has to be voluntarily accepted by both parties (licenser and licensee). if the license terms are unacceptable, the correct course of action is to not agree to the license.
I would not use any software that had such a restriction, even though I wouldn't be using it for a military purpose, out of general principle on two basic grounds -- 1) the authors are obnoxious moonbats who are abusing licensing to further a political agenda, and 2) it violates the fundamental principles of open source.
It is worth noting that even "loony left, anti-war moonbats" are against this bastardization of the GPL because exlcuing any class of potential users would set a bad precedent. In other words, they are setting the principles of open source higher than their own personal agendas, and I, for one, respect them for that.