If your bags were in fact left behind, I don't think it's unreasonable to ask for some sort of compensation.

I'm not sure, though, that asking for the overweight bag fee to be refunded is the best way to go --- the bag is still overweight.

The better way might be to write in to Customer Care (afterwards) talking about the inconvenience of having the luggage left behind as an AA/DL elite, etc. etc. etc.

I would expect that you would get more than $50 in compensation for that inconvenience --- and it would likely be in the form of discount codes to use for future travel (which may or may not be of any benefit to you, given that you fly AS infrequently).