What's that again? The service itself does not gain the site-owner's permission to display the links either, BTW.
They're using the service, therefore they have to agree to the terms of that service at some point. Also, if you're doing something to include content from a third-party, then you implicitly agree to allow that sort of thing.
The guideline you're invoking is intended towards a specific type of behavior that we see a lot and which is harmful. The fact that it is written rather broadly is intentional (I know, I wrote it), because sometimes we need to use them in broad contexts in order to get the message across to people doing spammy things unintentionally.
But those guidelines are not intended to be absolute and not meant to be used as "clubs" to enforce our way of thinking. That's why I called them "guidelines" instead of "rules". In an ideal world, we wouldn't need the list at all. I wish the list didn't exist. But we need it sometimes, so it exists for those purposes.
When you use a service, then you have to accept that services conditions. If you agree to their conditions, then you agreed to their conditions. A plugin can do anything if the user must agree to it first. In this case, if you're installing a plugin and setting up a link to that service, then you're agreeing to it, in my view. Since the plugin is not being deceptive, and it's clear that it's using their login services, then you are expressly agreeing to whatever their conditions are.
Note that links inside iframes like that don't confer google juice in quite the same way that, say, hidden links in footers and such do. That sort of thing is the reason the guideline exists. Using it as a club to make services "behave" according to what we think is not its purpose and not our original intent.