Lady Stefanie wanted to show this loser how she wanted him to deal with her feet as well as with her socks. That is why she chose to force him to do it according to her instructions. He had no choice but to do it her way because he was sure that the consequences would be dire if he did not. He had disobeyed her once and it did not end up well for him.
Lady Stefanie felt that she was not getting any real value from her slave. She did not want things to be like that going forward so she humiliated and dominated him for her own enjoyment. She cruelly used her sneakers as well as her socks to humiliate and to dominate him. He had no choice and he had to endure all that she threw his way. All he hoped for was that she would stop torturing him soon.