I work in class C space myself but I feel like you guys are in the wrong here.
Class D space doesn't mean that ATCOs should just give traffic information and hope for the best. A controllers responsibility regardless of airspace classification is to aviod collision and seperate traffic as long as neccesary. If **** hits the fan, 98% of the time the controller will be held responsible for the situation. So you can't really bash a controller for using the airspace wrong. You can always ask about the situation and have a constructive conversation about it, you should however in this situation (the way I've understood it) not call and bash him/her for using the airspace in a faulty manner.