Legalization of prostitution in Germany has not ended mistreatment of prostitutes by their employers, including forcing them to work too much, stealing their pay, and stopping them from quitting.

We should see this in the context of a general pattern of abuse of workers in fields ranging from fast food to manufacturing to construction to domestic service

It should not surprise us that this happens in prostitution also.

I don't think this is a reason for criminalization of prostitutes or their customers, any more than it is a reason to criminalize factories or their customers. Rather, governments must do more to prevent abuse of all kinds of workers. In some cases this requires additional regulation of employers designed specifically to prevent abuses.