You must be confusing us with the UK or France. You can carry (concealed or otherwise) knives up to 12 cm or folder that are not designed to be openable with one hand. If you want to transport a Chef’s knife on the metro, do it in your backpack, that is, you’re not supposed to have it at hand. Some types of knives (e.g. butterflies) are right-out outlawed, you’re not allowed to have them even in your own home.
Mostly though these weapon-free zones are a way to allow police to do searches.
The declaration of a weapons-free zone makes searches legal. If you don’t want to be searched, you’re free to not enter.