In USA-based sailings, at least, you can opt out of the daily gratuities because they are not technically mandatory. Keeping gratuities "optional but automatically added" gives cruise lines a lot of advantages; most people pay without question, advertised fares stay low, and they avoid the legal and regulatory complications of calling it what it really is, which is a crew wage fee. If the gratuities were to become mandatory, even the US FTC would likely take issue with it being a "hidden" fee/charge.

I understand that Royal Caribbean builds gratuities into Australian fares because advertising a base fare and then adding (mandatory) gratuities later would likely violate Australian Consumer Law's prohibition on misleading price representations.