Liquids on Domestic Flights

I don't believe there are any legal restrictions on bringing coffee or any other drinkable liquid into the sterile side of the airport. (domestic).

Individual security people sometimes take it upon themselves to go further than they are required to.

Eg - asking people to sip from a bottle they are carrying, or to check closely an opened container of liquid.

Not sure what you could do to challenge a security person. Maybe say "thanks" and walk down the corridor to the next security checkpoint?

Perhaps it should have gone through the x-ray instead and they were concerned that it might spill if put through there.