The following may be in part responsible for a seeming lack of anti-war songs in the USA in WWI.

"The Espionage Act of 1917 was passed to prevent spying but also contained a section which criminalized inciting or attempting to incite any mutiny, desertion, or refusal of duty in the armed forces, punishable with a fine of not more than $10,000, not more than twenty years in federal prison, or both. Thousands of anti-war activists and unhappy citizens were prosecuted on authority of this and the Sedition Act of 1918, which tightened restrictions even more. Among the most famous was Eugene Debs, chairman of the Socialist Party of the USA for giving an anti-war speech in Ohio. The U.S. Supreme Court upheld these prosecutions in a series of decisions. Conscientious objectors were punished as well, most of them Christian pacifist inductees. They were placed directly in the armed forces and court-martialed, receiving draconian sentences and harsh treatment. A number of them died in Alcatraz Prison, then a military facility. Vigilante groups were formed which suppressed dissent as well, such as by rounding up draft-age men and checking if they were in possession of draft cards or not."

from

http://en.wikipedia.org/wiki/Opposition_to_World_War_I