When I first went to the Old Slave Mart in 1984, there was an admission fee because, I was told, it was privately owned. I refused to pay and left, as I found it appalling that it was still run for profit. Disgraceful

Your preference would be that the workers there don't get paid? That would certainly be authentic.

All historic buildings and such require a lot of maintenance upkeep improvements and staff. Get pver it.

Outside of DC all museums charge admission fees. They have staff maintenance and utilities like any other business.