I get the ease of packaging things up so they can just hand out and go, but it could also be like the New Glarus releases where it's packaged in more reasonable quantities. They always have a two bottle allocation for the special on-site only releases, which is super easy to package and hand out. However in this case, Garagiste could package in two bottle increments and sell allocations like that: purchase two bottles, four bottles, or six bottles. It gives a lot more flexibility for the consumer and would likely drive more sales instead of the six or nothing method.
I would also reference Huna day and their tickets / allocations. You could buy the four bottle allocation, or if you want more, they offered eight or twelve. I never wanted more than four bottles, so that was always my choice, but I can certainly understand people wanting more and they had that option.