This is because, from the group, if you’ve added new hosts, you can simply reapply and will apply to all hosts again.
I’m very sorry that this was not as you expected, but it does have a purpose. If a host belonging to a group of systems is automatically added to the domain, it’d become rather more tedious to actually remove that host from the group simply so you can keep it from joining the AD. We’d see a lot more flak for that than the current method. Using the groups is relatively straight-forward anyway and hope you understand why we can’t have “group” add to domain.