I think saying clones don't take away from somebody else when it comes to quests, SITS, log-in prizes, ride-a-thons, and pod events is pretty short-sighted. The fact that the use of clones "floods the game with items" (your words) affects the economy in a very obvious way. For example, you reference that many of the log-in pins are pretty much devoid of value, but part of the reason why they have little value is the fact that you can get one for free on all of your unlimited accounts. Whether you care about log-in pins having value or not (and I don't, really), it's circular reasoning to say "clones make log-in pins worthless, and people should be able to use clones because log-in pins are worthless, anyways." Obviously, if the use of clones is directly related to one's ability to acquire items of value (and if the use of clones is unlimited), then it's advantageous for everyone to use as many clones as they can handle. If one person is able to become relatively richer by using clones, then another user becomes relatively poorer if they don't. So in that way, it does pretty directly affect other players. To your credit, you do go on to point out that everyone has the same ability to use clones (more or less true, although having multiple computers is advantageous in that regard, and mobile devices can't run multiple instances of the game). But I think it's worth considering whether we really want people getting on 12 accounts and podding in 4 rooms all by themselves. Even if everyone is allowed to do it, so it's technically "fair," it's still just kind of dumb... If staff wanted the game to be flooded with items, they could make that happen in a way that doesn't require cloning (increased Kevin spawns and increased rare item odds, for example). But overall, it's important to consider that "flooding the game with items" isn't necessarily a good thing, because it removes scarcity, and without scarcity, there is no economy.
Also, by your logic, playing mini-games against one's own clones should be permitted, since it doesn't prevent anyone else from acquiring items. And yet, that's pretty much the only explicitly and consistently disallowed use of clones.
With regard to ride-a-thons, specifically, I don't really see why they should be considered any different from regular host events. They require a queue, and the more clones are in the queue, the fewer times people who aren't using clones get through the queue. It seems like you just kind of made a distinction without a difference, there. If a hard cap was instituted on the number of clones permitted during host events, then the same rule should apply to ride-a-thons.
With regard to SITS, specifically, it seems like that actually pretty much depends on the use of clones. If we could only be on one account at a time, just riding the same ride over and over again for an hour just to get like 8 ash would be really frustrating and not remotely fun or worthwhile.
Overall, I'm kind of okay with what staff has been doing regarding clones lately: just telling us when we can't use them or when they're limited (BB8s, Kevins, Jedi Event, Minigames). It's still not entirely perfect (and unless staff devotes all of its time to enforcing clone rules, it probably never will be), but it's fine because at least everyone knows the rules.