Perhaps because they were angry over the cowards who continue to kill innocent noncombatants in Israel, Iraq, Russia...
Where did he find it...on a picnic table? Shoot there's nasty grafitti on every subject written on bathroom walls and public picnic tables everywhere all over the country. There's anti white graffitti, anti black, pro homo, anti homo, even stuff about yo' mama and yo' sista who will give you a good time if you call them!
The Muslims should feel honored...to be featured in graffitti here in America means that they have truly arrived on the American scene!
Your list is way too short.
Afghanistan, Indonesia, Pakistan, England, Australia, Saudi Arabia, USA, Lebanon, Germany, Egypt just off the top of my head. Set some time parameters so I don't go back too far.