This norm is terrible. It's a chief reason California tech companies become dysfunctional. Debate spreads common knowledge and ensures ideas get tested before execution. Hiding discussions is selfish, antisocial behavior that prioritizes your social image over idea quality and everyone's benefit. If you're working for someone who can't tolerate debate, leave.
It's weak leadership indeed that feels like debate undermines authority. A recipe for hell is
1. eradicating formal levels and hierarchy so everyone is "equal" and "ideas win", then
2. establishing a norm that debate never occur in public so that "ideas win" doesn't actually threaten whatever tyranny-of-structurelessness primate power dynamics emerge.
Thoroughly poisonous culture. A strong leader welcomes debate because his authority is secure.