The frothy stuff
is made by carbon dioxide (or nitrogen) bubbles. To make these stay sequestered within the liquid the liquid is kept at a high pressure. Not sure of the value of this pressure, but you try blowing hard enough into a bottle of coke / beer that the bubbles stop. Can't be done.

To water down beer you'd have to compete against this pressure with your own water pressure, depressurise it/mix it/pressurise it or what have you. It's not just a case of running a length of hose from a tap to a hole drilled in the top of the barrel or having a Y-Splice in the delivery tube going off to the mains water supply.

The only way I can think of them doing it on the cheap would be to have the tap alternate between beer and water feeds- but that'd need a lot of work to ensure no-one noticed.

If they really wanted to do it they could, but it'd need someone willing to put a pretty huge amount of time and money into doing it for the payoff of... crap beer. You'd take a good long while to make a profit of even the most basic versions of such a kit unless you were doing a really noticeable amount of watering down.
( uberdeity Wondered why Owen Lars knew of the term "hell" on, Thu 16 Apr 2009, 13:37, Reply)