Actually, Mr. Arpaio did not arbitrarily round up people, but when other issues caused police to stop the alleged offenders they were asked about their immigration status. The sheriff's department then handed them over to ICE.  A number of investigations by the Obama Justice Department cleared Mr. Arpaio, but there were always new accusastions. Very, very few individuals would pass such scrutiny by an agency with unlimited resources.  In this case Mr. Trump did the right thing.