Why not allow airplane passengers to bring small beverages on board?

Have you encountered this scenario? At the boarding gate in San Jose, Costa Rica, airport personnel conducted a second screening and prohibited liquids, such as coffee cups and water bottles. That's a first. It didn't make sense, as I had to discard a mocha and a bottle of water before walking onto the plane on Sunday morning.

Today, I sent an inquiry to U.S. Airways customer relations staff and wrote:

I travel often and, after going through the x-ray security checkpoint, sometimes purchase bottled water or another beverage to drink on the plane or fill a water bottle at a water fountain. Why would the gate staff prohibit beverages from entering the plane? Why make us depend on the flight attendants to remain hydrated during the five-hour flight? Thank you in advance for explaining that policy.

The airline replied this afternoon: "In response to your question, it may have had to due with weight and balance of the aircraft. If the load master indicates that extra items such as drinks are asked to be discarded they are."

Drinks, such as water bottles and in-hand coffee beverages, can affect the weight and balance of the aircraft?? It still doesn't make sense.