Sure they cant open every pkg or catch every illegal shipment but they don't have to look the the other way or play dumb when it ts obvious. I have personally witnessed this with packages that obviously contained weed. When brought to their attention they smelled it looked at it then through it back in the container. They Simply do not care and don't want take the time and resources to address this. These shippers know that FedEx will let these small shipments through the system and take advantage of it. In essence FedEx and UPS are the new drug mules across state lines.
I disagree on th weed, at my station, agents with a drug sniffing dog work the sort everyday. They pull any package that is supsicious, inspect it after the proper scan is put on it, and if nothing netted, it goes back on the belt. They pull anywhere from five to 20 packages a day. I don't know, and don't really care how many contain illegal drugs, but they are here everyday.