Are there any rules that say drinks purchased airside cannot be
brought on board? Should passengers generally be aware of this
prohibition? Or does this only apply to nonalcoholic beverages
purchased from the airside vendors, while the clearly flammable
liquors and performs purchased from the duty-free shops are allowed?

When we boarded our ICN to SFO flight, we were told that the US
government (presumably the TSA) had reached across the Pacific Ocean
demanded an additional security screening at the Seoul airport. Many
people had the drinks that they had purchased airside confiscated.
Everyone claimed that they thought these beverages, some still in
sealed packages, were allowed with their carryon luggage. At the same
time, other passengers were allowed to add their duty-free purchases
to things they carried on board; most of these contained alcohol and
clearly were hazardous materials.

If clearly hazardous liquids were allowed in large quantites, why
would they prohibit liquids that were probably innocuous? How would
they react to mother’s milk or baby formula?

This kind of arrogance coupled with the obviously illogical and
inconsistent application of some secret rules can only reinforce the
TSA’s well-earned reputation.