The only mountain bike pedal I've found that doesn't have spring recentering are Speedplay Frog Pedals.
I've tried Shimano SPD pedals and Crank Bros Eggbeaters/Candy and both have spring recentering in the angular float. It appears most pedals have a spring or other retention system that increases the tension as you twist your foot away from center until you've unclipped.
Are there any pedals similar to Speedplay X/Zero and Frog for mountain biking that have free float through its full range of angular motion?