Not all systems with ice belts in it are monitored 24/7. I observed the situation that systems with ice belts have no other players around for a long time BUT owned by hostile forces. The Ice belt is unused for days and vanishes. In NPC nullsec (yes, that exists) this situation happens often. May be because of time zones, don’t know.
You’re right about dropping on capitals. That’s a risk you must accept, of course. But this can also happen to any ship you place in a belt.
EDIT:
Forgot to mention that the idea came up because of discussions made in Ice Mining