There’s obviously some limit to your statement, right @Matt.Johnson? Most camps can’t/don’t offer every MB, so that’s a limit right there. Most can’t/don’t offer a large enough total quantity of slots to have unfettered access. That’s another limit.
It seems like there will always be some sort of preferencing system. Even if it’s not explicit, first-come first-served still would be a preferencing system. If the sign-ups are online, that preferencing system could become “the person with the fastest internet service wins”, and we (theoretically) start seeing services like eBay snipers to facilitate getting the MB choices you want.
I don’t know that it’s 100% compliant with the national guidelines, but it seems like, as preferencing systems go, rank and/or age is not the worst possible option. I personally don’t support “hard” limits that aren’t safety based. However, I can see that, if as a MBC I had a limited amount of time (e.g. one week at camp) to counsel a fixed number of scouts, I would want the scouts who are most likely to benefit from that instruction to be the ones who are attending. I can’t easily keep a waiting list, and punt the scouts who think that sitting at the back of the shade structure texting memes to one another in favor of the ones who were serious but didn’t make the headcount limits. Those scouts are already attending their back-up merit badges.