Thomas Shey
Legend
Having a premise to your campaign at all in the way you mean is a preference. Now, if you explain said premise before play begins and receive (apparent) player buy-in, then actively playing against it is not a classy move.
In the example I gave, you'd have had to explain it for character generation to even be done in a functional way. And yes, I usually assume people have at least avowed buy-in in a campaign's premise (the exception again tends to be on either my-way-or-the-highway GMs, or people with a bad case of Tigger disease, and both of those are problems in their own right).
And while I realize that "not having a premise" may be a thing sometimes in the D&D sphere, given that virtually all the Pathfinder campaign setups (you know, the thing they've been making most their money off of for years) have one, I can't assume its a not a relatively common one, and that's even just within modern D&D-adjacent games; outside of those and some fantasy games fishing in the same pond, I'd say its pretty much universal.