Emily is a foot fetishist but his lust cannot be contained to just one part of her body. Sure, he can tickle or whip her feet to get her going, but working her pussy into a hot, wet frenzy is not his only goal. He has to derive every bit of pleasure than he can from her body and that means using every little bit of the canvas that she has offered up to him. She has tits to be clamped, an ass to be whipped, holes to be filled and a clit to be vibrated. He cannot risk letting her focus too fully on her feet. After all, if she likes it too much she may get stupid. He would not want her thinking he cared what she wants. She is there to be humiliated and used, nothing more.
That is not to say that PD will not play with her feet at all. He is an artist, after all, and her feet are too perfect a subject to go untouched. They will receive an ever intensifying series of torments. A whipping is like paint, the spikes are like musical notes, and hot iron is like marble. He will fashion her suffering into a masterpiece. He just cannot miss out on that just because she may enjoy it too much. She is his to shape and mold, and through her pain he will make her perfect.