So you can’t use your own account outside your home otherwise you get charged? What if you have a family plan?
I'm sure their system recognizes what the main account is and who's the sub account(s). Sounds like a plan to push people to Premium charging 3.50-4 per head you're sharing passwords with.
If you're on Basic, password sharing with two people makes the monthly price $17. You're already paying more than the Standard tier for less features and at that point Premium's only another $3 away. They know they can't stop sharing, they're just going to charge you extra til it makes sense for you to play by their rules.