You should look at the reference file from your airline about travelling on AM mine says:
If I look at the rule for AM it says:
INLCUDES TRAVEL ON FLIGHTS OPERATED BY AERLITORAL FOR AM, AND TRAVEL BY AEROLITORAL EMPLOYEES ON (the airline I work for)

Maybe the Gate agent did not know what he/she was doing (there are more and more contractors at ATO)or did not liked you in any case I would have asked to talk to a supervisor.
I will travel on AM soon and I will print the reference file