There was at least one TSO who posted on this forum in the past about why pax can't be allowed to 'clear' their liquids by drinking in front of the TSO.

Supposedly TSA training teaches TSOs that there are certain deadly liquids that can be used to take a plane down. It is possible for a bad guy to actually drink one of those liquids in front of a TSO and not immediately fall down dead, even though the substance is toxic and will eventually kill the bad guy. However, since the bad guy is on a suicide mission anyway, he obviously doesn't care.

Of course, the names of these toxic explosive liquid substances that can be safely ingested without producing negative effects until a pax has cleared the checkpoint and finished his mission without falling dead are SSI. If you want to know what these substances are, you have to take TSA training.