Google apps is organization based service. Everything you do there is controlled and maintained by an organization. As Google+ is a social service, and social communication with an organization account is usually much more restricted than anything else(due to the fact that you get to share information with people outside the organization), using of Google+ by google apps users needs administrator approval.
You can go through the related considerations of an organization before they turn on Google+ service.
For the same reason, when you are creating a document using Google drive with your Google apps account and share it with someone who doesn't have an account of your organization, Google will alert you, asking if you are sure you want to share information with someone outside the organization.