That whole letter is about how much better the BSD license is than the GPL. If Minux would have been GPLd Intel would have had to open source the ME and therefore didn't use e.g. Linux.
I have not particular Problem with the BSD license (I use it myself sometimes) but give the facts that since years hackers are trying to convince Intel and AMD to relase their ME/PSP source code [1] (to safe us all from deeply infectable hardawre and unrestricted surveillance), I find it disgusting how Tanenbaum uses the situation to drive the discussion towards BSD vs. GPL and how right he was that the BSD provides the maximum amount of freedom to potential users.
In my opinion he should have said something like he knows that the BSD license also grants Intel the right to use it for whatever they want, but given the magnitude of the technological impact he supports the public interest to open source the ME and asks AMD to do the same with their PSP.
That way he could have used his role as the Minix inventor for the sake of transparency.
I don't think that Tanenbaum has any responsibility whatsoever to make that request.
The question about ME is totally, 100% orthogonal to the fact that it uses MINIX, and I'd go further and argue that this is sort of the point – releasing code as BSD is an explicit acceptance that it may be used in ways that one disagrees with.
There is no reason that MINIX could not have been used in the ME even if it were GPLd, for example…