Because bullys sometimes see these ppl as easy targets. The reason the bullys are bullys differs for each person, they might have problems at home or they might be getting bullied themselves. Or they can just be,, time-waistin-horrible-mean-brain-doning-psychos that feel better when everyone else is upset << thats usually the case anyway :P x ahahaha:)
He defo has twitter ,, just type in JustinBieber , it has like a tick next to his name to show its really him :)