The presumption by all of these vendors who operate their own app store is that they are legally protected in tying their hardware product (their smartphone, for example) to their online store.
But there is a history in both the written law and from the courts that makes these 'company store' policies illegal.
There is no technical reason whatsoever that prevents a smartphone/tablet vendor from allowing their smartphone/tablet from accessing anybody's online store. An Amazon tablet should be able to visit the Apple online store (if they sold Android apps) just as easily as an iPhone should be able to visit the Android marketplace (if they sold iOS apps.)
Imagine the outcry if Chevy could force you to only buy tires or gas for your Malibu from their store. Or if Mattel could force you to only buy outfits for your kid's Barbie doll from their store. Or if Dell could force you to only buy software for your laptop from their store.
The law has long been clear that this type of market coercion is illegal.
The only reason that today's device vendors are still getting away with this is because the threshold for an outcry hasn't yet been passed. But the more stories we read like this, the louder the volume is from the everyday consumer.