“Where there is no evidence of prior illegal entry"
That is the story buried in the lede. It is often said by advocates for illegals that there is no crime. Now you were careful not to say that but you phrased it in a way that can slide past an observer. If someone is lawfully admitted to the United States, that is if they present themselves at an official and open border crossing and after inspection by CBP they are allowed entry into the United States, then if they subsequently violate the terms of their entry, by overstaying for example, they will have not committed a criminal act only a civil offense. If however they cross the border illegally then they have committed a crime. What percentage of the illegal aliens that will be stopped by a LEO in Arizona for some other valid reason and who upon questioning are unable to produce any documentation as to their identity or immigration status will turn out to be persons who had been lawfully admitted and who just left their border crossing card at home or who overstayed a valid visa? The numbers will be so small as to prove insignificant especially when compared to the public interest in controlling what is in effect a mass migration from a combat zone. As was noted above any person who is detained for questioning who proves to be lawfully present will be speedily released, just as when the police question people around a crime scene. Persons who did not commit an illegal entry and who both overstayed their permitted time and failed to carry identification will suffer for their errors and the heightened scrutiny caused by the vast mass of illegal entrants. They should however have full rights to the protection of the courts and will not as I understand it be subject to expedited removal. Only an illegal entrant who is otherwise stopped by the police for a valid reason and who subsequently is determined to be both trespassing on US soil will be turned over to ICE for expedited removal. The idea that an officer must have some reasonable basis to effect a stop and question someone is not vacuous. The officer will have to articulate the reasons for his actions. That has been true for many years. Officers need better training on how to do that. If this reasonable effort by Arizona is frustrated then their next step could be to invoke Article IV, section 4 and declare themselves under invasion.
I believe that more officers should get properly trained and only a very foolish department would allow untrained officers to conduct the questioning.