Deduction of merits for deleted posts isn't enough because there are many low quality post weren't deleted, it wouldn't enough to downgrade the shitposter.
De-merit is better, the post don't have to be deleted and it can downgrade the shitposter, if someone who abused de-merit, that user will receive punishment or taking off their ability to de-merit.
I don't see difference between deduction of merit and demerit in context of this thread.
The discussion is about already deleted posts, whether earned merit of these deleted posts should be demerit or reverted. OP did not suggest to delete posts of shit posters, then demerit them. Two conditions come in orders, post deletion first, demerit / deduction of merit comes later.