... Changing rules around and making rulings when the rules aren't clear is not the same as allowing 3PP/fan made material. You can create house based rulings and still stay with in the official framework of current D&D.
Changing rules around is absolutely "allowing" fan-made material. In that case, the fan is you.
I agree that there is a disconnect here, without a doubt. We're dealing with a game system that tells you to change things and take ownership of the rules. The DMG has sections on custom options, most of which are examples used to illustrate how you can create your own content and rules. When you're in an "organised play" situation, you sacrifice options and flexibility to make your characters and campaigns interchangeable across tables and between cities, but outside of that sort of formal structure, imposing strict standardization on a game system designed around creativity is inevitably going to take away more than you could gain thereby.
There certainly seems to be the disconnect between generations pointed out by Morrus, but there's also a (probably related) disconnect between the way folks started playing the game. I started playing D&D with friends, at somebody's (parents') house. That evolved into dorm room/apartment situations, which were similar but without curfews and with a lot more alcohol. Now I play mostly using a VTT, with my middle-aged friends who merrily refuse to grow up. The point is, no one has ever told me how to play D&D - I read the books with my friends, and we figured out what we wanted to do with it. I imagine that someone who wanders into a game store and joins his or her first group in some kind of organized play scenario would have a very different perspective, if for no other reason than you would have someone "in charge" who would be describing how to play the game.
I think that plays into the whole idea of "allowing" material. I mean, in my experience the DM owns the world, the NPCs, and the backstory, but the group owns the campaign and the game. The DM necessarily has to organise and coordinate, and has a lot of authority, but isn't in a tyrant. Rulings are the DM's job, rules should be agreed upon. In my current campaign, for example, one player wanted to play Kieth Baker's 5e warforged... in Greyhawk. I figured it out, with a narrative that works with the campaign. Another player, though, wanted to go nuts with multiclassing--all within the published rules--and I had to push back with a house rule limiting multiclassing. In both cases, though, we arrived at a consensus within the group.
I've never done the organised play thing (the idea of spending hours in a folding chair at a card table under a fluorescent light doesn't appeal to me) but my impression is that there is a defined hierarchy, with some kind of event organizer and with the DM in a role that includes, to some degree, enforcing the rules and limitations of the event, for example the Adventurer's League. That enforcement role is, I suspect, a significant difference between the game store event and the at-home game night, and probably leads to very different perspectives on the issue of custom content.
