When I tour a house with a buyer that is because the seller has given me permission to do so. That permission does not extend to anyone who walks in off the street.
Newer agents do not always understand this and neither do people who see homes for sale and are curious about them.
The best way to see homes for sale is to call your real estate agent and set up a private showing or attend an open house if there is an open house.
A for sale sign is not the same thing as an open house sign.
I have had people knock on the door while I was showing a house. They had been wanting to see the house. I had them call the number on the for sale sign in front. I would have been happy to work with them but they were looking for a house for their son.
Often people tell me they are looking for houses for their sons or daughters but the sons or daughters usually are not at all interested in the house.