* It annoys me as a consumer.
* It reeks of incomptency ( yeah, go and disable that security check that looks like it is there to prevent malicious software to be installed on your device, yeah, yeah, that one ).
* It is a convoluted process.
* It leaves my device vulnerable. Let's not speculate (like you did apparently) but copy and paste from their own source:
https://www.amazon.com/gp/feature.html?ie=UTF8&docId=1003016...
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Update Phone Settings
Go to your phone Settings page
Tap Security or Applications (varies with device)
Check the Unknown Sources box
Confirm with OK
Open Downloads on your device by going to My Files or Files
Tap on the Amazon App file (Amazon_App.apk)
Tap Install when prompted
Tap Open to launch the Amazon Underground App
Use the Menu on the left and select Apps & Games
Yeah, I don't see anything about the "untick the checkbox again" part.