Marc,
Not sure what your main confusion is.
As we said above, "Eventually, Garmin will release software updates for the GNS and GTN products to make them compliant, but as of today they are not so you can't use them."
You can read the notice here:
http://www8.garmin.com/aviation/notices/1215.pdf
And that's all we know. There's no data as to why the GTN or GNS devices are not compliant, but as long as there is a service letter out saying they aren't, then we can't legally use one and claim compliance.
We also can't promise today that we'll be able to use the GTN or GNS devices in the future. Given that they are changing the firmware, they may make undocumented changes which make it impossible for a non-Garmin device to use them. We won't know until they publish the update and we can get our hands on it.
All of that worry is kind of a non-issue today though. Nobody needs a certified GPS in the USA until 2020, and there's a lot that can and will happen in 8 years. You'll get all the benefits of ADS-B with a non-certified GPS until 2020.