Having been in the position of the farmer in your tale, it is probable that he would have been more than totally exasperated.(My mate said that he was totally exasperated with her when she let the unleashed dog through the gate after reading the warning notice!)
When you devote your life to the welfare of animals, you don't want to be forced into making the decision between shooting someone else's animal and allowing it to continue to damage your own animals and profits. It's a dreadful place to be and it feels entirely unnecessary when they wilfully ignore a sign.
Edited to add: Sorry, Colin, it's (hopefully) obviously not a poke at you as I think we agree anyway, it's just that particular experience is a little, shall we say, raw for me.