Wow, I hope this is rhetorical. No, you never give anyone your password for anything. Yes, you absolutely let them go through the dull process of setting up new accounts.
> When I move into a house, do I want to spend the first day looking up manuals online to try and work out which buttons to hold down in order to claim the devices as my own?
shrug, yeah, why not? That's the kind of stuff I would do anyways. Moving into a new home requires all kinds of "oh, ok, that's how that works" kind of discoveries. "Hmm, the breaker panel says '#5: kitchen' but that one also apparently has the garage lights. But not the garage door opener, for some reason."
> not be able to easily set the heating until they've moved in ... the epitome of "first-world problems"
I will concede that this is truly a PITA and a drawback of modern life.
Computers are so cheap that we can "afford" to put them everywhere. Hopefully the next generation of these devices will have sane fallback/factory reset modes that behave like their ancestor devices until they've been setup/enabled.