I gotta disagree with those being the ONLY reasons someone should ever be booted from a party; while CM certainly has very lax gear requirements, and people should be allowed to watch the cutscenes when it's their first time, booting someone for poor gear in dungeons such as AK or hard mode primals is absolutely reasonable. (The trick is, they're adding hard-coded gear level requirements for these anyway, so my point is purely academic)
Once you hit level cap, you can no longer make your character more powerful through the accumulation of EXP. Instead, you do the equivalent of leveling up by acquiring better gear. Attempting to walk into HM Titan with only AF and subpar weapon and accessories is like trying to walk into CM at level 40 or less. You can't do that because there's a hardcoded level requirement, because you'd get roflstomped by the trash mobs if you tried. It was simply an oversight on SE's part to not treat gear level the same way, which they are going to correct in patch 2.1