Every argument made pretty much applies to most of the new Cisco offerings or any other embedded Linux-based system.
None of it is new and at first glance I think all of the arguments are flawed.
The source download for each release is still there (just checked). When you select the image you can see the link for the Release Notes and right next to it GPL Archive.
Note that it says GPL Archive not full source. They only provide the GPL packages of the code used and modified as part of each release. You won't be able to have a full and working system with the non-free code missing.
The EdgeRouter for example makes use of a proprietary development kit to take advantage of the Cavium Octeon network CPU. They are likely under NDA by Cavium (or a company that develops software for them like 6WIND) to not release details or source code which is why it's not the source of the same kernel (it actually likely is the same kernel just all the non-free modules are missing).
As for the U-Boot question. You would really need to dig up the details a bit more than a letter to support.
One of the things people forget is that the author (copyright holder) of GPL software is still able to offer the software under other licenses (e.g. non-free). If I write a piece of software and release it under the GPL there is nothing that prevents me from also selling a GPL-free copy of it to someone else so that they can use it without the restrictions of the GPL.
In the case of U-Boot on Ubiquiti devices they have indicated before that the release of U-Boot used is provided by the vendor SDK for the hardware platform used (e.g. Cavium) and not something developed internally. If you suspect that it is in violation of the GPL your best course of action is to notify the copyright holder, not make accusations, as the copyright holder may have granted a license other than the GPL to someone in the chain.
This is a lot like how Vyatta switched from Quagga (which was a fork of GNU Zebra) to ZebOS to get non-free features like multicast routing support using a drop-in replacement. ZebOS is actually based on GNU Zebra but uses a different license (IP Infusion was founded by the authors of GNU Zebra who decided to make the code non-free and go to market. They were able to do this because they held the copyright).