Spammers are incentivized to use the witnessspacee, therefore removing a spam filter will only give them more opportunity to spam.

Witness data is counted at 1/4 of the data in the legacy part of the transaction. OP_RETURN lives in the non-witness scriptPubKey, so even if you allow arbitrary large OP_RETURN each byte still cost 4 weighted units, whereas the very same but pushed into the witness costs only 1 weighted unit.

Core knows this. They are compromised.