Ignoring the fact that we've had prisoners, in places like e.g. Guantanamo, on and off who were/are held for decades without ever even being charged with anything, China has also charged its prisoners with crimes.

You don't agree with China's decision to make practicing religion a crime, I don't agree with the US's decision to make possessing drugs a crime. Both are "crimes" punishable by incarceration according to the two governments.

Presumably China has also convicted it's forced labor of some crimes as well.

Regardless of what the laws are, I believe that prison labor is a bad practice with big ethical issues—it incentives incarceration as a means to procure cheap/free labor that can be used to turn a profit for those in power.

EDIT: To clarify: I don't think giving prisoners an option to work is a bad thing, but nobody should be allowed to profit off of such labor other than the prisoner themselves. Anything else tends to lead towards undesirable incentives and outcomes.

Criminals are still humans and we should be using every effort to rehabilitate them and make them valuable members of society and contributors to the economy in a far more significant way

The Gulags were also full of people who have committed crimes. Some of those crimes were even non-political!

I can't say that's the argument that I'd use to defend slavery, though...

So slavery is legal as long as the slave is a criminal? I'm not following what's being implied.