It's quite likely that those are compound charges, meaning possession is not the only charge, i.e. they are arrested for something else, then found to be in possession.
So they are charged disproportionately because they commit crimes disproportionately,[0] and some of them happen to have a bag in their pocket while doing so.
[0]Aboriginal and black Canadians are grossly overrepresented in Canada’s correctional institutions
http://www.academia.edu/5110524/Race_Crime_and_Criminal_Just...
This paper is itself biased against Ockham's Razor, they hit on some truth here, We cannot discount, however, the probability that increased rates of offending among certain racialized groups contributes to their overrepresentation in correctional statistics. but then spend the whole paper trying to hand wave it away.