Others seem to be commenting entirely from their own perspective. Try viewing this from the TSA’s or an attacker’s perspective instead and thinking about boundary layers.

In regard to the comments about disposing potentially hazardous water bottles, a TSA agent can rightfully assume that an attacker could dump the contents of said water bottle at any point before the security checkpoint, including into the public water supply. The fact they don’t attempt to prevent this from happening doesn’t make them full of shit, it actually saves tax payer dollars.