I don't blame the contractor. I blame a poorly drafted statement of work, or unclear work instructions, or poor management by whoever engaged the contractor.
A contractor will only do what they are told or paid to do. In most cases, they are not being employed to think, and they may have only the barest idea of departmental or company policies and standards.
I have seen a few examples of badly performed graffiti removal lately, and I think it is all down to badly briefed contractors. RailCorp eventually painted over graffiti on a bridge in Leichhardt, but the painter failed to paint over about 25% of the visible graffiti. I presume what they missed was not in their statement of work.
Energy Australia employs contractors to paint over graffiti on their substations, which they generally do quite well, but they never tell them to remove the graffiti from signage on those substations. You often end up with a nice, freshly painted wall with graffiti-enhanced signage plastered all over it. The point is to remove all the graffiti - 100% of it - not just the graffiti on the brickwork.