Before Microsoft pulled the upgrade, I tried to install it on four perfectly-functioning domain-connected computers. It failed every time, causing old issues (explorer.exe immediately crashing every time it starts) and new ones (start button ignores mouse clicks, Cortana fails, start panel blank). In each case, I was forced to do a clean re-install to get build 10586 to work.
Since Microsoft is not explaining its reasons for pulling the upgrade, we are entitled to speculate. Here’s my guess: Restricting the upgrade to Windows Update is a way of delaying it without having to publically acknowledge pervasive problems with it. I suspect that they have temporarily stopped providing it via Windows Update as well and that they are madly trying to fix it before anyone notices the delay. Clue: the upgrade is not available via Windows Update on any of the 25 Windows 10 computers I administer, even though build 10240 was installed on them months ago.