Ok, here's a suggestion... have a single system, but make it work like this....
Take the average amount of hours/days that all accounts play each month. Divide $15 by that amount and you get your hourly/daily rate. Now just make it so that once a person hits that average playtime and therefore $15, they aren't charged anymore money.
If the average playtime of all subscribers is around 30 hours, that equates to 50 cents an hour.
Lets say Ronnie plays 13 hours: Ronnie pays $6.50 that month.
Now lets say Josie plays 118 hours: Josie pays only $15, because she has exceeded the maximum billing.
I doubt HT will change their system, but if they did, I would think something like this would make sense without changing for the worse how much they charge their loyal customer base. while maybe encouraging busy people who don't have much time to drop by every once and a while and not feel like they are losing money in their subscription. And honestly, they can set their hourly/daily rait however they want, but the important thing is they have a maximum amount somebody has to pay.