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Because the apps can initiate a call by just pointing a webview at tel:0000. Another reason is hybrid apps, imagine a contact hybrid app running in something like PhoneGap. You want to click a button and call right away without the phone asking you to confirm your action.

I'm not saying that Apple's design is good, but it is documented.

That was my thought as well. I'm sure the decision was made to connect as seamlessly as possible, without the user having to stop and click a dial confirmation.

I imagine the other apps are responsible for their behavior. The browser gets special locking down because practically anyone can end up sending you anything.

Other apps can too. You have to use telprompt instead of tel in the URL scheme.