That's not something Apple can control without removing features from Mail that exist in literally every modern e-mail client. Essentially what is happening here is Mail is rendering a website. It's a very small website and it's been designed to look like Apple's UI to trick you.
So here are Apple's options:
- They could disable HTML / CSS completely, and push Mail back into the dark ages.
- They could offer a toggle to disable HTML / CSS in Mail, which few people would use and would cause unexpected issues when a valid e-mail requires HTML / CSS to render.
- They could disable specific HTML like FORMS, which would prevent this particular scam but again, cause unexpected issues when a valid e-mail has a valid form.
- They could scan the email for specific html like FORMS and provide a notice/alert that the email might be attempting to steal passwords. This is probably the best scenario but even so it would scare users away from legitimate emails using forms (which granted, are very few)
But again... this e-mail would look the same and FUNCTION the same whether you viewed it on iOS, or OS X, or Windows, or via Safari or Chrome or Opera... whether you loaded the email from Mail.app or via iCloud or Gmail or Outlook or any other email client.
And any "fix" Apple takes on its end is really only a bandage. It wouldn't prevent this phishing email from functioning on other e-mail clients and any "fix" they offer has downsides as listed above.
It's not an exploit. It's not a bug. It's not something that can only affect iOS users outside that it vaguely looks like the iOS environment. It's not a "Meta tag issue" or the result of some faulty programming on the part of Apple's iOS development team.