Unfortunately, the response from AA is correct. Original Routing Credit is a "favor" from AA, something not guaranteed -- technically, you only get miles for what you fly. In practice, AA will be willing to provide original routing credit when its their fault you didn't fly the assigned routing -- misconnects, mechanicals, etc. Even weather, sometimes... However, if its your fault, its pretty rare to get credit, and with standby, pretty impossible.
Think about it: If you could get original credit for direct standby, you could book a complex four segment routing to some destination, go standby direct, and then get the original routing credit for those four. Thats obviously not a good situation.
As for gate agent attitude, I have encountered that attitude on occasion. How late are you adding yourself to the standby list? In my experience, gate agents tend to get a bit more snippy as it gets closer and closer to departure, especially when the gate agent has already started processing stand-bys...