"Since they're implementing this in a stupid way, it's perfectly valid to exploit their method[...]"
Really?
If you went to buy something on Amazon, and found they had a "stupid" vulnerability you could exploit in order to get half off of your order—maybe some Javascript hack that made the part of their system that calculated the price you pay think you actually ordered a smaller version of the product—is that immoral?
Is leaving your house or car unlocked a sufficiently stupid vulnerability to become "perfectly valid to exploit"?
Your examples break federal law; I'm exploiting Amazon's proprietary codebase to lower my prices / someone is still breaking and entering into my home. Those aren't moral because federal laws are being broken.
If Amazon charged $20 for a book if I were to buy it on my laptop, but $10 for that same book if I buy it with my phone, why in the hell would I buy it on my laptop? How is that immoral? I'm presented with two options: $10 or $20 for the same item. The company has offered me a contract of payment and I am to choose one, or I can take my patronage elsewhere. This is not a matter of breaking into a server and SQLi'ing until you can make an item free; this is the company offering me something for cheaper, depending on how I buy it.
Really?
If you went to buy something on Amazon, and found they had a "stupid" vulnerability you could exploit in order to get half off of your order—maybe some Javascript hack that made the part of their system that calculated the price you pay think you actually ordered a smaller version of the product—is that immoral?
Is leaving your house or car unlocked a sufficiently stupid vulnerability to become "perfectly valid to exploit"?