Sorry but I call bullshit.
Gasoline in Nevada from the pumps themselves to the gas in the storage tank is regulated. You can learn more here: http://agri.nv.gov/Index_Measurements_Index2.htm
Or here: http://www.leg.state.nv.us/NRS/NRS-590.html
Long story short having bought thousands of gallons in gas from some extremely remote locations across North America, more often than not the problem is in your own vehicle / tank and not with the station. If a station in a semi-remote place like the Empire store did open up their storage tanks and somehow dump water in there to get a 50/50 mix, the locals would know pretty damn fast, the State of Nevada would know pretty damn fast, and that store would know they screwed up pretty damn fast when the cash registers start collecting dust and their petroleum distributor cut off their supply. Sabotaging fuel in a place like that is economic suicide.
Sorry, while I want to be sympathetic, and trust me I'm not really a fan of the Empire Store either (during burn week it's a safety hazard due to traffic congestion), this is one claim I just can't help but yell bullshit on.