It's a nice package but the paying premium member is required to move outside IH for PM with non-premium members.
Want to exchange emails with someone? They need to have an email account, a client and internet access. Want to talk on the phone with someone? They need to pay for phone service. Want to Facetime with someone? They need to be on an Apple device, and be on a LAN. That's how it works.
And, your claim is inaccurate. Anyone including non-Premium users can PM anyone else during Happy Hour on Fridays. Anyone can acquire the feature if they want it available on demand.