In many states, they already can for felonies: https://en.wikipedia.org/wiki/Citizen%27s_arrest#United_Stat...

What they lack is qualified immunity, so it's pretty rare because the risk of being wrong is too great. Which seems to be a valuable take away.

You know that's perfectly legal in California right? You can make a citizens arrest for a misdemeanor in progress or a felony you have knowledge of.

And suffer the penalties of kidnapping if they do it for the wrong reasons, yes. This would do two things: one, people would have a huge incentive not to do that, and two, maybe we'd raise the threshold for jail and raise the penalty for wrongful imprisonments, meaning cops don't jail people for victemless crimes and we don't have such a massive prison population.

Also, we could make laws saying that prisoners are only allowed to be kept at approved prisons; a law that could be applied equally to police and citizens, while ensuring the government gets to approve who gets imprisoned.