Amazon debuts a new Appstore app, features Amazon Coins

> Instead of seeing a dollar amount associated with their purchase, app developers show the purchase amount as coins. This could take the sting out of spending cash, Amazon has believed.

I'm fully aware that Amazon is not the first company to be doing this, but does this not break contract law? They're actively trying to trick people into spending more cash for a product than they rationally would.

Like, where's the line between this and just offering people a joint before they sign your contract, so that they feel easier about spending money?