The Water Menu is around for more than six months already and is another detail to differentiate the First Class Lounges.
Why don't they serve just tap water? Because they don't wanna appear cheap. If you wanna sell 10,000 USD-tickets, you wanna give the customer the feeling that he is getting value for money. It's about the end-to-end experience. You might have your favorite whiskey brand and you might have your favorite water brand.
Last night you dined at a high-end restaurant and they poured Hildon. This morning at the FCL, they pour the same. It's the hidden codes that you are belonging to 'that circle' and that's why you fly LH and not UA.
Let's hope they axe the Smirnoff Red in F soon forever as I am pretty certain that no one flying First is drinking that stuff anywhere else...