It does because the bulk of the spam takes place by regular users who use their regular wallets to broadcast their spam tx to regular nodes that would then decide whether to relay them or not. When the nodes reject nonstandard transactions, they won't be relayed to reach miners and the bulk of spammers will not go through the backchannels to contact miners to mine their nonstandard not-relayed transactions.

We can see that clearly by analyzing the chain and seeing that for example the standard rule on OP_RETURN size does indeed work. https://blockchair.com/bitcoin/outputs?s=time(desc)&q=type(nulldata)#f=recipient,type,time