The rationale of the directive was that you have bought your ticket from a TA and paid a service fee. For that reason, they should be providing you with the full service you paid for, which in this case includes processing the upgrade. TAs do have access via AC's res system to process this on their own without using an AC agent.

Calling an AC agent takes time and thus removes the agent from processing AC customers, thus it does cost money for AC to process your call, and nothing to process the TA's (as it is done by computer).

While this cost saving may appear inconsequential to you, if 100K upgrades are processed, at about 2-minutes a call, that translates into a lot of both time and $ for AC.