We see Dr. Sloan constantly ask his detective son to put out APB's (All Points Bulletins) or BOLOs (Be On the Look Out) to bring suspects and witnesses in. When found the police just walk up to people and place them in the squad car, taking them to the station for Steve and Mark to interview even though it's pretty clear they didn't want to come. If there is no probable cause for an arrest or an active arrest warrant, the police can't make you go anywhere against your will. Once in a while, a wealthy or educated person will assert this and ask a lawyer to be present but this is rare.
Dr. Mark Sloan often undertakes actions which would result in evidence and investigations being compromised. Since he has never been a police officer, these actions could result in cases being dropped or overturned upon appeal. There is no way that a District Attorney would allow Sloan and his colleagues to perform the actions that they do in the show.