But if we’re talking about a law that actually says the cop cannot take your phone no matter what, and they do, then any public defender would be able to point it out and the judge would certainly have to enforce it. I can’t think of a way the cop would abuse their power because, in this case they don’t have it.

they can abuse their power because they're a cop, with a badge and gun, and the right to maim or literally kill you with it (and probably get away with it even if it's not strictly legal) if you don't comply with their demands in the moment. again: cops consistently do not care about or follow legal procedures they're supposed to, frequently fuck up those procedures even when they do, and most cops probably don't even think of it as their job to secure some airtight case that stands up to legal scrutiny. it's not a profession that lend itself to the kind of charitability that's being given here, and the record of the profession makes it even less deserving of that charitability.