The regulation of alcohol in the US is a mess, involving both federal and state authorities, and a web of rules meant to make it hard for smaller players (and in the case of some states, out of state players) to compete. Alcohol is "taxed" (there are various types of what one could describe as a "tax" due to a governmental entity in the case of alcohol) at both levels (there can sometimes be more than two in case of county and/or municipal levels) -- BUT those taxes/amounts are, in most cases, small, especially when we're talking about high-end wines/Champagne and what first class service is supposed to be about. There is, however, reporting/paperwork associated with these payments (at all the relevant levels). It may be that the non-US airlines want to avoid having to deal with that paperwork because it is more costly/risky/annoying than the actual payments. Regarding the inconsistency between airlines, destinations in the US (for what it's worth, IAD is in Virginia, which has stricter alcohol rules than some states) and even the same airline/destination but different flights, I would attribute that to 1) rules differing city by city, 2) airlines having different internal procedures to handle this matter and 3) crews applying (or perhaps, not applying) the guidelines given to them by the airline differently crew to crew. I'd be willing to bet #3 is the key driver of variability.