Logically it makes sense but I guess there will always be nodes with little integrity who would accept the unwanted transaction.

I'm saying: make it harder to spam, not impossible. If a transaction is rejected by some nodes, it is more likely for it to be outpaced by the widely accepted transaction. So in order to make sure it gets into a block, the spammer might need to increase the fee. Especially if he's in a hurry. What am I missing?