I think the problem now is that in general good quality HARD maple is far more expensive then good quality birch (about 80% more expensive). However, in general birch is much more uniform in density and hardness when compared to maple. Therefore you can get low quality and cheaper maple easily in US which ends up in plywoods. The birch plywood that Portenzo used was high quality. Also P&Q is using high quality birch plywood either from baltics or russia. Portenzo's statements about maple's higher hardness are partly true since it can be harder then birch but in this case its meaningless. Even more so WWII Mosquito fighters where made out of birch plywood so if its ok for that then I assume it can take the punishment it gets when used as iPad 2 case material. All in all, I think Portenzo just wanted to get easier access for raw materials. In order to do so they had to choose soft maple as raw material which then resulted lower quality wood. The fact remains, just by looking at it its very hard to tell apart good quality maple and birch plywood. In this case its easy since their current maple plywood seems to be low quality.
While I am no wood expert, I have purchased some wood for home projects before. I believe the problem has more to do with consistacy of quality control when making the plywood as well as selecting. I look at it kinda like produce and this may not be a very good example, but when I worked in that industry you had to hand pick quality. You could not just pick up the phone and order up 300 cases of perfect lettuce. I think the same is Tue in wood.
While there should be some sort of consistency in the wood, not every sheet is going to be of the same quality I assure you no matter if be Birch or Maple.