As I understand it, Pemberley was what would now be called a "stately home". The public rooms were open to any tourists who wanted to see them (when it was convenient), and the housekeeper could expect to make some money from the tips she got showing people around, in the same way you would go on a tour of a famous house in England today. It was perfectly acceptable, and had nothing to do with whether the visitors were acquainted with the owners or not. Actually, I'm not sure about this, but I think tourists would only be allowed in when the owners were not at home. From Sense and Sensibility, I seem to remember that the faux-pas was wandering around the house uninvited when the owners were at home.