A US military sniper is not going to accept a mission to assasinate a US political figure, and they are not going to accept any mission that requires them to attack a target in the US. Period. If you want that job done, you find a police sniper...
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Pardon me for paraphrasing Battlestar Galactica, but at one point in the series, Adama was asked to use the military as police. His response was that the military was used to defeat the enemy and when used as police, the people become the enemy.
Granted, police snipers are not killbots who pull the trigger simply because someone said to, but we are constantly arming our police more and more to become a military-like force in an of themselves.
If the general populace cannot have the same level of weaponry available as the military, neither should the police. What a cop can use, so should I be able to (although I think what the military can use I should be able to as well, but that's a whole different ball of wax).