Second, it's not offensive in the sence that young people would find it offensive(swearing, nudity etc).
Thirdly, it's in his signature, you're not forced to read his sig and it's not part of the comment he is making in his post. He is also not forcing you to do anything about it, he is only saying to impeach them. If I had in my sig 'Jump of a cliff now'.... would you? Most likely not.
It's more or less free speech, it's his own opinion and he could yell it out on the street and technically not get arrested unless for noise complants, disturbance etc...
And lastly..... 'Sticks and stones....'
