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So this is kinda tricky.

On one hand: what you said. I don't believe it's a breach of the license, because an attribution necessarily involves citing the author's name (or username, in this case) - §4.c is about false _endorsement_, not simply "this person wrote this". Something that implied that imported users wrote content specifically for this site would fall afoul of it, but a use of a username doesn't. That said, I can understand the concern about folks not wanting their names used without their permission on a site they know nothing about. 

On the other hand: firstly, I'm _required_ to display those names. The footer of every Stack Exchange site says "content licensed under (license) with [attribution required](https://stackoverflow.blog/2009/06/25/attribution-required/)". Part of the blog post that links to says:

> So let me clarify what we mean by attribution. If you republish this content, we require that you:
>
> 1. Visually indicate that the content is from Stack Overflow or the Stack Exchange network in some way. It doesn’t have to be obnoxious; a discreet text blurb is fine.
>
> 2. Hyperlink directly to the original question on the source site (e.g., http://stackoverflow.com/questions/12345)
>
> 3. **Show the author names for every question and answer**

Emphasis mine. Point 3 there is unambiguous: I have to use the names.

Secondly, if we assume for now that this display does fall afoul of BY-SA 3.0 §4.c: graying the names out has no effect on whether or not that's a breach. Color can assist user recognition and experience, but it has absolutely no legal standing for this sort of thing. It'd need a different user display entirely with affordances that clearly lead you to find out that it's not a native user. Which brings me neatly on to...

Thirdly: it's technically difficult. This is deliberately third because that _shouldn't_ be an obstacle where it can be avoided, but this is the real world and I have to balance ideals with practicality and the time available. There's no easy way to find out whether a user is "native" or SE-imported: it's doable in small numbers, but going over a large dataset as is necessary to show the user list, it's horribly inefficient - and that's already an inefficient page. Likewise, adding a separate display for native vs. imported users adds an extra cost - a relatively small cost, but nonetheless an extra thing to develop, test, and maintain, and an extra thing to process.

So - it's not the answer you wanted, but short of someone finding a unicorn solution, I'm going to have to leave it as is for now. I hope that gives you some insight for why, at least.