I hope I don't run into this Gate Agent, cause I would have the concourse supervisor come down so that this gate agent could be re-educated on the policies.

The rules state that Medallions and First Class passengers can use the boarding lines to bypass the general boarding line. Yes, some might see it as elitist. However, the folks on Virginia Avenue have decided to make this a perk for those that have earned/payed for it. It's not up to the gate agents to decide whether or not Delta's corporate policies are unethical, and if they do they can bring it up the chain of command to try and get it changed. They are not to deviate from a policy though if they don't think it's fair. Delta is Southwest - not all passengers are equal - there's a reason that some passengers get priority boarding, free upgrades, priority check-in, etc.