It's also a game which targets a much younger demographic than most any other TCG on the market. I'm not discounting that there are allot (and I mean ALLOT) of brats who probably bug their parents to buy then shiny new cards, but it's a little different asking a kid to fork over $80 for two cards than a teenager. At this point producing a Shaymin EX tin, especially around the holidays, is TPCi printing money and moving some old packs.
* A lot
Sure, tinning will help more people have access to Shaymin EX, but what you are saying is that if a good/shiny card comes out it should be tinned to appeal to the lower demographic. I disagree with this. The fact that some cards are rarer then others makes opening packs and trading for cards fun.
Also, what is the difference between brats who bug their parents to buy them new shiny cards and paying 40-80 dollars for Shaymin EXs? If they can already have access to their parent's funds like you said why wouldn't they be able to afford 2 Shaymin EX from their parents?
Being a competitive player means you'll have to spend more money anyways on trips, food, and cards. The people who do well, 6 to 36 in age, all travel a ton and play test with every deck and can afford multiple Shaymin EX (or at least have access to them). The younger non-competitive demographic you are talking about has no idea really how good Shaymin EX really is (let alone what its worth), and would easily trade it away for a cooler looking card, like Charizard EX, Rayquaza EX, etc.
I'd agree with you that printing Shaymin EX in tins would be good for Pokemon economically, but I don't think it will matter too much. They are already printing 3 legendaries this Fall: Rayquaza, Latios, and Hoopa. The younger demographic, unknowing of how good the cards are will easily go for the Pokemon they like most.