I've seen lots of people carry on drinks in cups, also, but I could see it might be problematic as people could easily spill them. In fact, some people do. If they were in a cup that had a way to close the lid completely (not paper cups, say you had a travel mug), I don't think they'd care, though. So I think it is somewhat unsafe, exactly because there is jostling and fighting over space, etc but have never seen it forbidden on my flights.

There have never been rules about limits on what is checked baggage in terms of liquids. You can't have explosives or flammable stuff. You could pack a whole bottle of wine, after all, if you wanted.

I didn't know diabetics needed special coffee, though, but they shouldn't care about that packed in baggage. I did see someone get busted for trying to bring into the US some weird branch with seed pods and leaves or something like that at the airport upon arrival. The dogs smelled it in their baggage.

I've read that airlines hate cleaning up after people who bring meals and drinks with them onto the flights, but, a) the airlines created the situation and b) I can't see that they do much of any cleaning at all.

If the liquid can be carried in a cup, it's not hot enough to be a safety issue. The juggling-your-stuff-as-well-as-a-beverage issue, well, it is what it is. I can't say I've seen anyone spill anything.