That's not entirely correct. For example, at the University of Münster you do need to pay a semester fee ("Semesterbeitrag"). Granted, this will get you a ticket for all public transport (except IC/ICE) within northrine-westphalia which is definitely worth more than what you're paying.

But yes, public university in Germany is really, really cheap.

You can also often apply to have the semester fee waived due to financial hardship or special circumstances, though I have no experience with that.