Saltpeter has a long history of medical use. It and other nitrates were employed by doctors during the 18th century to treat such varied problems as asthma, sore throats, and arthritis. Not that it was used wisely for these purposes, however -- physicians were far less knowledgeable in those days, and potassium nitrate's hazards were not understood as well as they are now. Saltpeter can be toxic in large quantities. It can cause anemia, methemoglobinemia (a blood disorder), headache, stomach upset, dizziness, kidney damage, and can raise one's blood pressure to a dangerous level. It is thus strongly recommended against, and no one in his right mind would dose himself or anyone else with it.
Given that saltpeter doesn't suppress sexual urges and is dangerous to use, the Army lacks any reason to administer it to those within its ranks. Yet those two facts will probably do little to dissuade those who are certain the coffee continues to taste funny.