From my studies, I have arrived at the following conclusions.
Snipers and Scouts were indeed employed as members of the regular army. However, Guerilla tactics were, at the time, considered uncivilized and un-gentleman like conduct.
Keep in mind that during the Revolutionary war a British sniper decided not to take the shot that would have killed George Washington because a shot to the back was considered cowardly at the time. The ‘skirmish line’ tactics were bred out of the revolutionary war.
Keep in mind Confederate sniper Charles Grace’s famous shot at Spotsylvania,
which killed Union General Sedgewick at nearly 1,000 yards, is officially recorded as the actions of a member of the army during the conflict.
Many ‘Partisan Rangers’ operated on their own and were not official members of the military. Quantrill’s Raiders and Confederate Sympathizer Sniper Jack Hinson (mentioned above) would fit into this category.