There might be a more benign reason for it. In GDPR countries, if you turn it off they will probably need to delete all the biometric data. If you then turn it back on again, it will have to regenerate all the biometic data and re-scan every photo. If people toggle it too often, it's going to consume a large amount of CPU time.
You can confirm it by using an open source facial recognition tool, like the one built into Immich. Importing photos takes much, much longer if you have face recognition turned on.
Of course a more sensible way to do it would be to allow the user to toggle it whenever they want, with the caveat that if the turn it back on, it might take a long time to start working, or might only apply to new photos after the initial back-catalogue freebie.
Or they could just be being dicks.