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I would expect that the answer to your question is yes to both of those.
Shooting at Mexican nationals across the border, even if they fired first, is a pretty much guaranteed bad thing. The politics are probably the main reason, but the possibility of hitting an innocent, or some one who would suddenly become an innocent are just too high. I am personally getting a little tired our continued appeasement of a government that sees the US as a safety valve for their internal political and economic mis-management.