Yes, it is very subjective. That is why the standards orgs (ANSI, ISO) have developed objective (can be physically measured) levels for sound that have been shown in exhaustive field studies to correlate well with subjective responses of humans - annoyance, interference with reading and speech comprehension, sleep, etc. HUD and many state agencies have model ordinances based in these objective standards that are recommended to local (city, county) political subdivisions, which have the final authority on what levels will be allowed, when and where. Some local public entities go it alone and cling to subjective descriptors in the ordinances that are not conducive to fair and consistent enforcement, but most do have objective levels specified in the codes and ordinances.
AMA is not a controlling authority, and cannot anticipate the actions of local entities that do have the authority. They tried early on (the 90 dBA @ 9 Foot rule), but it was based on rules applicable to specific communities and countries (ex. GB has national standards) and could not sensibly be generalized for all locales. Further, all model flying venues are not created equal, particularly as to distance from noise sensitive sites (residences, schools, hospitals...). A decision was made by the AMA Sound Committee circa 2000 to drop the fixed level that had been recommended and instead advise clubs/members to look up the applicable local noise codes/ordinances and do what it takes to comply with them.
Some AMA competition rules do specify sound level limits, as has been noted. The rationale is very sensible if you think about it. Lower permitted levels means more venues available to hold events, and importantly, practice for them. Look at pylon racing for an example of a class that does not (probably can not) have sound level limits. Few model flying venues can accommodate them due to noise levels generated. Ever notice in MA articles reporting on them that the class is perpetually dominated by just a few guys from Nevada?