Comment # 7 on bug 997322 from Josef Reidinger

(In reply to Michael Chang from comment #6)
> Why don't you work out by yast2 or editing /etc/default/grub directly to
> disable os-prober but rather insist to invert the default if no explicit
> GRUB_DISABLE_OS_PROBER is set, and thus affect existing running
> installations. It works for your intention but may not be the case for
> others.

I expect issue Olaf have is that yast2 set this value at the end of
installation, so if any part of rpm post install script call grub2-mkconfig, it
will use default and default is to disable os_prober. So it is by default
disabled on SLES, but only at the end of installation.