Police are given the latitude, with some limitations, to arrest people legally even without charge. Importantly, they must release uncharged persons within a certain timeframe which seems to have been respected in this case.

If ultimately there is no charge and the person is released, they can obviously contact a lawyer and have access to the courts.

We can debate all this as a matter of public policy and law. What we should not do is sacrifice our credibility by pretending that people are being kidnapped and "disappeared" or comparing this to what is going on in HK.