It's marketing. In Europe they started with invite-only, then they opened invitations in the user accounts (up to 10 users) and finally it was open for everyone.

They also changed the way the accounts worked along the time. First your account was unlimited time with voice ads, then that was only for first-time accounts and new accounts would only have 20h month services and nowadays everyone gets only 20h month unless they pay for subscriptions.

I bet they'll do the same in the US but since you have some other services they will prolly modify the offers a bit.