I do think it's ridiculous to push any writer to write in any particular way, but I think it's also important to understand the reasons for these categories because like or not, this could decide a lot of things in terms of how well your book gets exposed and sold
Another point I meant to raise is that we've seen a number of discussions here about what actually constitutes science fiction - with Star Wars routinely condemned as fantasy rather than SF. My perception is that the regular SF readers want to see some form of focus on the technology or science involved, no matter how speculative, and when I think of most of the big names in SF this is something they aim to deliver.
So if someone is writing in a SF setting, but they are not focusing on the tech and science, IMO they are not going to go down well with those same SF readers. It may be argued they are a minority, but with SF readership being relatively small in the traditional market, I'm not so sure. And while writers such as Bujold have been highlighted as an exception, I would treat her as an exception and not the norm.
In which case, if the setting is science fiction, but the focus is on characters, then IMO it's better to market such work at a wider market - such as YA, where this appears common - as opposed to the small adult science fiction market.
The Hunger Games IMO is a classic novel, but I don't think many regular science fiction readers would accept it as a science fiction novel. Similar for The Fifth Wave trilogy, Divergent trilogy, or the Red Rising series - all of which have SF settings, but all the focus is on character rather than the tech or science involved, and have been hugely success in the YA market.
(Similar arguments could be made as to why 1984 and The Handmaid's Tale are not marketed as science fiction).
IMO that's the market pointer right there for deciding whether something is better marketed to SF or Y - especially when writing about young characters coming of age in a science fiction setting.
[Anyway, I've made my points so I'll shut up now. ]