Maybe they don't support Apps accounts because they know they can't enforce the Real Name policy on it.

Scenario: George Kibble from ABC Company has an Apps account from his company. He signs up for Plus, but puts in 'The Duke of Perl' as his 'real name'. Google cancels his account... Including his email! Now George not only can't use Plus, but all his correspondance for the company is lost, forever!

It would take only a few of those incidents to start a business riot.