Given that the coupon was clearly designed as bait for price matches, I don't fault anyone for asking, and asking repeatedly. But why should Amazon bother matching most little $3 games that barely anyone will buy (which seemed to be a lot of the requests)? For the most part they matched the big ones that would sell the most.
Keep in mind, Amazon doesn't just have carte blanche permission to set prices to whatever match point they want, the providers of the keys can place whatever restrictions and stipulations they want. And I have actually heard an indie dev say in a past sale that Amazon price matched his Steam sale price without his permission, and he wasn't very happy about it. Different sites negotiate different deals and digital goods are a lot different than going into Target and asking for a match on a TV.
This would all be totally valid, had Amazon not pricematched awesomely last Summer Sale.
And I'm sure there's something about pricematching in the fine print. Indie devs are just more likely to not read said fine print. I hardly think Amazon went out of bounds when they pricematched a dev's game.