It's quite common to go to open houses without an agent or to ask the sellers agent to show you around. What that sellers agent was telling you sounds illegal,

It's quite common to go to open houses without an agent or to ask the sellers agent to show you around. What that sellers agent was telling you sounds illegal, and I'd be very surprised if they wouldn't show you the house if you insisted on not having another agent there. They have a fiduciary duty to get offers and show all of them to the seller they are representing.