I have a half-baked theory about advertising in customer-centric marketplaces (e.g. Amazon and App Store):

What if advertising is just like drug use? Most hate it and blame the suppliers, but there will always be demand for it. So, what if the marketplaces have a choice: Provide a "legal" way to advertise that is "safe" or outlaw advertising and push the behavior towards a "dangerous" black market.

Before App Store ads, app makers definitely used "black markets" to drive downloads... and now with the App Store ads I've heard rumors the app ecosystem is overall more safe.

What if it's the same for Amazon? By giving sellers a "legal" way to advertise, Amazon makes customers and the marketplace net safer.