Lol. That's how I've been playing too. I just place them at the sealed hate until I hear something different. But the cultists don't come through the gate. They are simply there trying to summon. So maybe it does make sense.
The official ruling is that yes you still place Cultists on closed gates, so yes Dagon would place Cultists on gates that had been sealed. It takes a lot to deter a fanatic of the Old Ones after all, and a sealed gate just isn't enough.