Great company.

The chance of finding a powerful hero in the dungeon is lower than in the tavern, but not by much. Also, the heroes found in dungeons are often already somewhat leveled up. So, if you ban recruiting from taverns, you should also ban recruiting from dungeons.

Firstly, you need to overcome the dungeon's defenses. Unlike the tavern.
Secondly, the dungeon might contain completely weak heroes.
Thirdly, finding a powerful hero in a dungeon depends on finding a dungeon with a Logistics hero on week 1, either level 1 or 5. On the second week, it's not as interesting, unless it's a high-level hero with perfect skills.
Fourthly, a Logistics hero won't provide an immediate, significant advantage, like Luna/Jeremy/Galtran would.

All of this is much less problematic than the hassle of firing a hero. It's better for dungeons to provide a bonus than to ruin games, like dungeons with Mullich in The Succession Wars did.

But this is just my opinion; ultimately, the organizer should decide.