I don't see the problem here...The got more pre-orders than the stock they will received at launch date, so they sent email about delays in the shipping. Seems pretty normal if they are just now received they stock and noticing it won't be enough or if the shipment will be late. You still going to get your discount but just going to get the game late.

They are not trying to make you cancel, they just suggest that if having the game day 1 is more important to you, than you can always cancel the game to go buy it else where at launch. They are just simply telling you that the order is not set in stone yet and you can still cancel.

I think you are a little too much paranoid on the situation.

What I would do though is, if you paid for prime to get release date shipping, to go on support and ask some sort of compensation for it. Usually they add a month to my prime or 10$ discount on a next purchase.