Does the one pi zero per customer limit of the shops apply per order only or is it one in your lifetime? There are so many uses for a pi (zero), that such a restriction is a sad thing.
I'm not a representative of either a retailer or the Pi Foundation, so I cannot for certain say this has nothing to do with the latter -- meaning, it is a policy of individual retailers, not the manufacturer -- but I think it is most likely the individual retailers.
That being the case, I suppose some stores could try and impose a "one zero per lifetime" cap -- but that seems a bit absurd. It also means that there may be some retailers which have no limit at all.
There is at least one popular retailer that I've noticed apply a "one per customer" limit only with regard to the zero when ordered without accessories for $5, but no such limit when ordering them packaged in a kit (and they did not prevent me from ordering a total of two at once this way, one of them in a kit).
I notice the same retailer obviously sets some of its stock aside just for kits, since they will show the plain zero as out-of-stock but various kits including a zero as in stock.
I think most retailers who sell them alone for $5 probably go out of stock very quickly, and if they do impose a limit, it will probably be "reset" when they restock. I.e., I'm sure when aforementioned popular retailer gets another bunch, I will be allowed to order another one without having to get sneaky, create a new account, have it mailed to a friend's address, etc.
I believe this is all tied to production runs. The Foundation has a couple of "official" retailers they work with publicly -- in fact, these are more than just retailers as (at least in the past) they have actually produced the Pis themselves. I know there have been other more localized relationships of this sort, e.g., there was a red model B at one point only available in (parts of) Asia, produced under licence by a distributor there.
I don't know what the relationship of the smaller/unofficial retailers is, including whether or not they have to go through one of the Foundation's two international partners (RS Components and Farnell element 14). I imagine at some level everyone not licensed to do production themselves must pre-order a certain amount and this is in part what determines the size of a production run.
Official statement by Raspberry Pi Engineer & Forum Moderator:
Pi 0 and Pi 0 W are still under a one unit per customer sales limit due to high demand, and the need for production capacity to build other models.
This limit is not likely to change in the near future, although efforts are being made to improve the supply situation.