Because by convention they have set clockwise moment as positive. We are taking moments so we consider the sign of the direction of the rotation not the load.
Inspecting line you have highlighted: the sum of moments about B, we see they have assigned negative to the reaction C (-7.c) implying counterclockwise as negative.
So with this convention, the moment 5*2*1 should be negative because it is counterclockwise.
It would not make any difference in the final reactions, it's just that usually we assume the counterclockwise moments as positive.