Apart from the fact that opening (one of your many) homes to the public for a fee is a bit vulgar, this commercialisation puts the royal family on a par with businesses offering the stereotypical quaint British tea experience
People have been opening their homes to visitors for a couple of hundred years (there's an example in Pride and Prejudice, altho that was not for the plebs to do, they'd have to be happy with an Open Day for the tenants). Teas make good sense - walk around the place for an hour or two,then sit down,take the weight off your feet and have some tea and nice cakes. Puts you in a good mood for buying souvenirs. and rounds off the day nicely.
I'm sure the RF and all the other peers like the Devonshires and Carnarvons who open their homes will be devastated to hear that it's a bit vulgar, though.