Do you know they didn't witness a crime? I will say i don't know if they did or didn't. Him entering the house (which is on multiple videos) is a felony by Georgia law. If they DID witness him enter, they were totally within the law to perform a citizens arrest.
According to reporting, the senior McMichael’s claimed he was in his front yard and saw Arbery run past. He said he fit the description of someone suspected of burglaries in the area. That is not witnessing a crime.
Like TK said, that is according to reporting. So that’s what we have to go on.
Other reporting also says responding officers were ready to arrest both McMichaels after the initial investigation but the DA told them to stand down.