ANYTHING that would fail to support good order is prohibited.
From what I gathered from your link, it is mostly about “sleeve” tattoos. The recruiter asks about tattoos the minute you sit down and talk to them. The tattoo this young man had is on the back of his shoulder which would mean it would never be seen while in any kind of uniform.
Quite frankly while I agree that rules must be followed, it does seem to be that the military has become extremely PC, to the point of insanity.
I read the policy and Q&A at the link you provided... twice... but I stil can’t find the part that prohibits tattoos of historical American flags on one’s shoulder.
A few years ago they'd have probably given him a tattoo waiver. But not these days. In this lousy economy, more and more young (and not so young) people are walking into recruitment offices looking for a job. The Marines, like all the branches of the service, are having no trouble filling quotas and, in fact, are turning people away for disqualifiers that were once previously waived. It's tightening up. The armed forces are becoming very selective on who they enlist (and who they allow to reenlist).
The days of the military being that last "fall-back" option as a guaranteed job are over.