The military does many things, but it is not responsible for prescribing the meanings of English words.
Words are defined by their usage. This word is used outside the military, and in these uses, it does not denote a person with special qualifications; it denotes a person who does a thing, i.e., a person who snipes.
http://www.dictionary.com/browse/sniper
http://www.thefreedictionary.com/sniper
http://dictionary.cambridge.org/us/d...english/sniper
http://www.collinsdictionary.com/dic...english/sniper
http://www.macmillandictionary.com/u...merican/sniper
http://www.oxforddictionaries.com/us..._english/snipe
http://www.ldoceonline.com/dictionary/sniper
As you can see, the essential sense of "snipe," as a verb, is to shoot at people from a concealed position. The original meaning, since folks seem to want to talk about the original and proper meanings of words, predates modern sniper doctrine by at least 100 years.
To find a definition that made a sniper a qualified specialist, mosquito had to go to Wikipedia. Do I need to explain the difference between Wikipedia editors and dictionary editors? Hint: one role requires nothing more than an Internet connection and a room temperature IQ.