I am by no way condoning throwing things at the linesman (assistant referee/flagboy/benson/whatever), but it seems these fans hadn't had anything to celebrate for a while - apparently you can't insert Arsenal jokes here anymore because they have Ozil !
And fair play to the fan. It's a great shot. And the linesman didn't roll around on the floor like he'd been shot, he just didn't flag for offside again, but it was worth it.
Just imagine if these fans got to celebrate winning a trophy.