The answer seems obvious:because they have cut it . Another thing is the reason why they cut it.
To explain it, we are going to go back to Classical Greece, in the times of the Athenian general Alcibiades (450 BC–404 BC) . In addition to being a great soldier – he came to fight with the armies of Athens, Sparta, Persia and, again, Athens – he was a great statesman and orator, perhaps inherited from his grandfather Pericles.
Alcibiades he had a great defect, or virtue others would say, his ability to make powerful enemies. They took advantage of any trifle to attack him, as happened on one occasion when he cut off his dog's tail. His friends, who also had them, reproached him for his attitude, since he himself "threw more fuel on the fire."
The general's response:
That is what I was proposing. As long as the Athenians mess around with the dog's tail, they'll leave me alone and make no inquiries about my other actions.
Today the expression «the tail of the dog of Alcibiades » has remained as a phrase to designate the things that public figures do or say to distract attention and avoid talking about things that are more important or compromising for them.
Source:The anecdotes of politics – Luis Carandell