That isn't the contact. You need to watch it again.
I know still images are problematic with this stuff, but I hope the below helps.
At this point in the image, contact is about to be made. Baccus's left foot is on the ground, and Pittman is going in on it. Baccus's right foot is swinging - and you're correct, he does kick Pittman's shin - but that is not the main contact and is not what Pittman is sent off for.
Pittman's studs make first contact with Baccus's standing left leg. That's the offence.