It is mandated by SOLAS that ships hold a muster drill within 24 hours after leaving port. However it is only "encouraged" that passengers attend this drill.
On passenger ships, an abandon ship drill and fire drill shall take place weekly. The entire crew need not be involved in every drill, but each crew member must participate in an abandon ship drill and a fire drill each month . Passengers shall be strongly encouraged to attend these drills."
The cruise lines have made attendance of the muster drill mandatory. So it is a matter of semantics. Be it SOLAS or the cruise line's requirement it is still a requirement.
After 30 some cruises it does get a bit old. It is also nice now that most cruise lines don't require that you bring your life jacket to the muster station. I've tripped over a few trailing straps from people who drag them up the stairs.