Mistress Anna wanted to collect her shit because she felt that it was what her slave needed. That is why she chose to do it on top of his head and once she was done, she sat down and had him eat it. She was in the toilet as he ate it so that it would be easy for him to clean up after himself when he was done eating it.
This loser deserved to be dominated and that is what the mistress did. She turned on him and she made him face her toilet. She then pooped on him and she enjoyed dominating him for fun. The mistress had a great time torturing the slave and making sure that she got what she wanted from him. He was scared of her and what she was capable of and that is why he did all she asked him to do.