It would be pretty terrible if my security cameras stop capturing video streams because the iPad would be out of battery or taken somewhere else.
If the only problem here is "But it could be moved!" then solve that problem.
(Of course, that is not the only problem. We can't get to that point to begin with because Apple is Apple.)
Even if the iPad recognizes it's always plugged in and discharges from 100 to 80 occasionally, it's still a use case that increases the chance of battery swelling.
I don't mean this as "Apple knows best, psh, silly customer" but a bit of "that's one reason this exists" and a bit of being annoyed that you can't run tablets battery-less off AC to prevent it.
It would pretty obviously be easy to add a wizard that prepares the iPad to be repurposed as a stationary home hub, setting the battery to be held at 50% and neither charged nor discharged, sending a notification to your phone if it's unplugged, choosing a slideshow to double as a digital frame, etc. But to be fair to Apple, no other companies would build such a thing either, it's just the consumption oriented system we live in.