The point is that it doesn't matter if it's free or paid, if what you pay is near-zero cost. It definitely cannot be a factor of ruling out spam if the fees are too cheap and aren't an adequate deterrent.

And even "adequate deterrent" is wrong, as a term, because a script kiddie might not afford a good spam attack but a deep-pocketed adversary may not be deterred by the costs, because by attacking in this fashion he is getting side-benefits by harming BTC.