In law there is a concept of malum in se, and malum prohibitum. Malum prohibitum is the policy layer, where nodes regulate the network, malum in se (bad in itself) is the layer of strict validity.

Because of the basics facts about information theory it's impossible to deterministically ban spam at the validity layer, you have to do it based upon rough consensus at the policy layer, which is how it's been since Satoshi.

Its just an engineers autism that leads people to believe that if things are not deterministic then they are not valid. Perfect is the enemy of good enough.