Why can’t I turn off auto-updates for the Add-on Manager?
It is deliberate. The Benecaster Add-on Manager is what downloads, checks and installs your add-ons, so it updates itself automatically to make sure security fixes reach every site. On Plugins its Automatic Updates column reads Auto-updates enabled with no Disable link.
Your add-ons are different: whether they update automatically is your choice, per plugin, in that same column. A site-wide setting that turns off all automatic updates still applies to the Manager too.