The airworthiness limitation section is the only section in the maintenance manual that is FAA approved. That makes anything in it mandatory for all. I don't know if it is the same in other countries but it probably is in many. The reason for this is that the legislative rules for making regulation extend only to government agencies, not to manufacturers. If manufacturers could make things actually be mandatory we'd be in a world of hurt by the time their legal departments were done with the manuals. Traditionally, all overhauls of components are not listed in the ALS and thus are not mandatory unless you are operating under some form of FAA approved operations specification that specifically makes them so. Also, airworthiness limitations lists only ever get longer, never shorter. Calling a service bulletin mandatory is a misnomer but gives the manufacturer some psychological recourse in a court. If they really wanted it to be mandatory they would petition the FAA to make it an AD which I'd venture has happened but rarely.