I believe it helps avoid legalities involving gambling and the such as well as helps to keep the money circulating in their marketplace.

Aside from keeping that money locked into their ecosystem, If you buy coins from az with cash, all the coins in circulation, essentially is an interest free loan of dollars to amazon, which as long as coins are in circulation are never fully repaid.

Then you have the gift card problem, where some percentage of the, coins in this case, is never spent, free money to amazon.

Because ot creates confusion in the mind of the buyer who will then not realise how much they are spending in that ecosystem.