From Amazon help:

(Not as bad as you think).

Learn More About Ads On Kindle and Fire Tablet

If your device includes ads, sponsored screensavers appear on your device's lock and home screens.

Amazon is able to offer some device models at a subsidized price by including personalized advertising on the device.

Kindle e-reader display ads as a screensaver when your device is in sleep mode and on your home screen marked as "Sponsored." On Fire tablet, ads display on the lock screen. Tap on an advertisement for more details. Connect your device to the internet to receive the latest offers.

If you originally purchased a device with ads, you can unsubscribe by making a one-time payment to cover the difference between the device price with and without lockscreen ads. You can find more details on how to unsubscribe from lockscreen ads on Help page

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