The cups aren't being sold (or at least that's not the intention). The cups have a deposit on them.
Reusable cups are usually a bit thicker and higher-quality than cups intended for single use. And depending on what type of cup you replace with what other type of cup, you may need a lot of refills to compensate:
While this is apparently part of the implementation of the EU SUP directive, this particular law is Belgium-specific. (EU directives need to implemented in local law in a specific time frame, but the timing of specific measures varies across all nations of the EU.)