Chomsky usually defines anarchism [1] by saying powers and hierarchies have to justify themselves. Keeping people safe can be a reasonable such justification. The point is just that such powers should never be taken for natural, and always be questioned. If jailing doesn't probably make people safer, then it loses its justification.
That being said, there are certainly situations where people with anarchism (or anarchism-like) beliefs have essentially jailed people. For example YPG/YPJ forces in Rojava certainly took IS/ISIL forces they won over as prisoners.
PS: Never ever heard of an anarchist that thinks the US prison system is anywhere near of justifying itself though, but that's probably something all radical leftists agree on.
I don't think you have to even be left of the centre to realize the US jail system had serious flaws and need major reforms.
Just looking at the incarceration rates and the number of reoffences compared to international levels show that something is completely off.
Huh, that's not what I've heard. Anarchists I've seen talking are very strictly against explicit hierarchies, and prison obviously is very much that, since bare minimum you have prisoners at the bottom of the hierarchy there.