Anyone who has traveled with a baby knows they can easily scan liquids inside the bag (e.g. formula and a gallon of distilled water). There is no need to even put them in a separate plastic bag. Sometimes they do a bomb residue test, however.
I suspect throwing out liquids was a scam to sell you more drinks at higher prices after the checkpoint.
I travelled through the US from Mexico to Europe last year. Bought a nice bottle of Mezcal at the airport in Mexico, had it in a sealed bag to enable travel. TSA agent didn't trust the travel seal, so he took it out of the bag, scanned bottle with some cotton swabs put into a device (my guess is HPLC), it was OK so he re-sealed my bag with TSA tape so I could carry it on multiple flights in Europe.
I suspect throwing out liquids was a scam to sell you more drinks at higher prices after the checkpoint.