When I found out that the mobile ones are free, I was really shocked, 6$ for the normal one and free for the phone? I thought they would make it a 2$ app or something.
I think they basically want to encourage everyone to have them because it saves them a ton of headaches dealing with hacked accounts as far as recovery, gold selling, spam, etc. If they didn't have to pay to manufacture them, I bet the physical ones would be free too.
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...and if you disagree with me, you're probably <insert random ad hominem attack here>.