Honestly I don't see anything wrong with it. If that's illegal then you'd have to start saying knee bars are illegal to, and so on.
You shouldn't have your feet planted anyways really, I suppose I learned from boxing for 3 years but even today I'm in MMA and I was taught to avoid keeping your feet planted. I slugger may have his feet planted but if your on the balls of your feet your able to move with it and avoid damage.
^^ Bring on Pettis/Henderson 2! Henderson can't run from him, or he will get kicked in the face again.
Fedor Emelianenko is the Greatest P4P MMA fighter to ever walk the earth, hands down.