Users have the option to prove an XMR tx happened using a view key.
No they don't.
The viewkey is only available to the sender. It's the receiver that wants the "proof".
No it's not.
The receiver received the funds and they can use their view key for their wallet if they need to. The viewkey shows all the incoming transactions of the address it belongs to. I don't see an issue here. Maybe you don't understand how what it's purpose is?
https://www.reddit.com/r/Monero/comments/3mknh9/mymonero_and_viewkey/cvg7oc3/EDIT: WTF, how did I get in a dash thread? Dammit stop throwing links in these two threads to drag everyone into this bullshit never ending godamn argument.