Because we, Americans at least, are lazy/sloppy with our language. How come the word "gentleman", which once only meant an owner of land, gentry, but has sense come to mean the ideals of such a person. Or "Cavalier", which used to be a military unit, but came to be understood asa person who displays the same attitude of these military men. Or "decimate", which originally meant "to reduce by one tenth", to now be synonomous with obliterate. Or why does "Bad" mean good? or "cool" mean socially suave? "Poach" has slid into the morass of slang.

To be honest I always viewed poaching by the original definition with the addition of shooting game out of season. What can I say? I'm lazy/sloppy too, I guess.