I doubt they intended something like this to be possible, but what remains to see is whether or not they'll be OK with it. I wouldn't be surprised if Apple disabled :hover and other interactive CSS features within Mail.app in a future update.

Apple has blocked apps for a lot less than providing a feature that intercepts users' email and email password to circumvent Apple's own sandboxing and inject content into their mailbox...

What could they do about it? Not allow you to create a mail account that points to linkedin.com as the server?

Apple could yank the certificate that LinkedIn is using for configuration profiles, which would make installation significantly more difficult for the average user.

Apple, to my knowledge, has never revoked a single cert for config profiles since they're rarely used outside of the mobile dev market.

Any examples of them doing what you have proposed?

Uhm... Apple doesn't whitelist nor blacklist services -- read: TCP connections to whoever you want. LinkedIn, in this case, is well within Apple's TOS/EULA.

I think you're confused about the policies on their app store.