This is the way it was 10 years ago. Open mats were for paying members to come roll with other paying members, no instruction. Drill if you wanted to, but most just chose to roll.
Seems like in recent years I've seen gyms advertising its open mat as "BJJ is one big family, no drama here, we train with everyone, etc." I assume with this change in ethos also came the idea that open mats should be free. If you want to roll with folks from the crosstown gym, you'll have to keep it free. Not many are going to pay $10-20 to roll on a Sunday afternoon at a different gym.
Charging a fee is a time-honored way of limiting scare resources (the scarce resource being the mat.)
So it is like a golf club that would let other paying members from other golf club drop in for a game in Sunday.
But most Bjj gym are business and not social clubs.