You are correct that all of the features on the website are premium. Best in Slot was the main free feature, but that changed with The War Within. We talked about it in this thread:
There is only one type of account on the website. Users need to make an account to use the free trial as well. We make use of the account to save user settings - we’ve tried in the past to allow doing BiS without an account, but we couldn’t get the experience to be what we wanted without somewhere user-specific to save data.
It takes a lot of work to maintain this site. The changes to the game in Midnight were some of the biggest changes we have ever seen. We’ve taken this opportunity to make huge upgrades to the back end of the website. The entire website is once again running on a simulator, except now we do simulations in real time, since our new simulator is so fast. We do not want to run ads on our website, so the subscription model is best for what we are doing.