Although Dong Tianyu was somewhat averse to Cave's dismantling of humanoid creatures, he had no objections to sending other humans into the Hundred Ghosts Banner.
Some might say Dong Tianyu is being pretentious or hypocritical, but that's the reality. After all, for Dong Tianyu, even if the Hundred Ghosts Banner devours many human souls, it's just a tool. If he no longer wants to use it, or if it can't keep up with his pace of growth, it can be replaced or discarded at any time.
But Cave is different. For Dong Tianyu, Cave is not just an ordinary tool; it can't even be considered a tool, but rather the ace up his sleeve that he relies on for rapid growth, second only in importance to his own life.
That is precisely why Dong Tianyu did not want Cave to dismantle humanoid creatures, as he subconsciously believed in his heart that doing so would be akin to him directly consuming people.
Therefore, unless absolutely necessary, Dong Tianyu would always adhere to this unwritten rule.