Ok, this makes sense. I must have been using a client that had 'Zap' icons and because I didn't have a Zap balance I just assumed the buttons didn't work because I had no balance.
I didn't realize every single person who downloads a zap-supporting client had to also set up a separate app to handle the micro-transactions and host that app somewhere where it is available 24/7 in case their post gets zapped.
then you can't be zapped
you can only be zapped if your profile has a tag in it that tells people where to send your zaps to
without that, a zap button shouldn't show up, and if it does show up, it shouldn't do anything