My question involves police conduct in the State of: Arkansas

The other night my friend had a party at this house. I was standing outside (within 5 feet) the front door which directly faced the street talking to a friend. He had a beer in his hands as he was talking (he is 21) and neither of us were being over loud. We saw three police officers walking down the street and when they saw us they were directly in front of the house. The three of them then turned and came up to us.

Note: There was a music. It was loud enough to be heard from outside.

They immediately questioned my friend if he was 21 and when he procured his ID they then demanded to see the owner of the house. He opened the door and quickly went inside, but when he tried to close the door one of the officers placed her foot in the door and then threw it open. She did not then enter the house, but stood right on the doorstep. When my friend came outside to talk to her, he asked if he could shut the door and the officer said no because then she would be unable to see what is going on inside the house.

The rest of the story is superfluous, but they just told him to make sure everyone at the party was 21 and to be respectful of his neighbors. This coming after one of the officers admitted no noise violation had occurred because the music could not be heard outside of 100ft.

My friend has had problems with police officers walking around his house before, with the last time two other people sitting in the back yard talking (not visible from the street). They were startled when police officers emerged from the darkness.

Sorry for the length, questions: 1. I realize I am ignorant of the laws but do Police need some type of probable cause to walk around his house, or come up to the door, or demand to speak to the owner? (I can understand if the beer or music was PC enough). 2. Can they actually require the door to stay open the entire time? I understand the logic behind it, but found it odd.