[AnthonyMouse]

It enables competition, which keeps everyone honest. If Amazon had an app store which was known to be full of malware then people would stop trusting them, but when there is real competition they need that trust to make money, so they would put in more effort to prevent that. If you find that your platform's app store is full of malware today when there is no competition, what is your alternative? Throw away your phone? Never install any apps?

The exposure is also not any worse when the people you trust are equally trustworthy. If you install ten apps from one distributor or ten apps from ten distributors and all of the distributors are equally trustworthy, the chance of an app you installed being approved even though it was malware is the same. It may even be lower because the distributors have to worry more about their reputations when there is competition.

And it also applies the other way. Right now they can reject apps that you want not because the apps are malicious but because they compete with the distributor's own. If there were five other trustworthy distributors then you could install it from any of them. So you could always get e-readers that compete with Amazon from Google, search apps that compete with Google from Microsoft, web browsers that compete with Apple from Mozilla and so on, no matter what kind of device you have.