Why does LEGO state the number of parts on US boxes, but not in Europe? I was reading Huw Millington's review of set 70700 Space Swarmer on brickset, in which he states:


  The box is almost square and this being from the USA, like all US boxes but unlike European ones, it states the number of pieces on the front. I'm guessing that it's some sort of requirement to do so in the USA, but I wonder why LEGO don't print it on European boxes. It's useful information for the consumer, after all.


Which I think is a very valid question.

Are there indeed such requirements in the USA? Are there other countries with similar requirements?

And I guess this will be harder to answer, but why doesn't LEGO just state it on all boxes anyway?
There is indeed a requirement that packaged products sold in the US must indicate the weight, volume or quantity plainly on the package. This is regulated by the FTC, which says in its requirements,


  The net quantity of contents (in terms of weight or mass, measure, or
  numerical count) shall be separately and accurately stated...


Canada has a similar law that says the net quantity of packaged products must be on the label.

The European Union's product labeling requirements do not appear to make the quantity a mandatory part of the product label. I do not think it actually prohibits putting the number of pieces on the packages, so it may not explain why The LEGO Company leaves that information off their European packages.