In practice they do. Yes spammers can pay more or go to more effort to go direct to rouge miners but that does not mean we should abandon all attempts to limit spam. Your second paragraph reveals the problem with your thinking. It is the nodes that are in charge, miners should respect node runners preferences not the other way around. Nodes Mempool filtering (which they have always done) is a signal of our preferences to the miners. Thanks to miner pool centralisation, individual rouge miners are able to ignore the nodes wishes and put unwanted junk in the blocks. The fact that mining has become this centralised gives us bigger problems anyway but that's another issue. If we accept that nodes no longer have a say in the content of the Mempool then bitcoin is dead and we might as well let the big two mining pools run everything. Then we have become ethereum or any other shitcoin.