Player touches out of bounds

[mcrowder]

I understand IP (borderline, but technically correct) if he's out of bounds when the ball is snapped and steps forward to catch it.

But what's the rule that makes this a foul if he touches the end line before the snap, and readjusts in bounds also before the snap? I thought I'd read on here before that in FED, a player that leaves, but is not substituted for, is still a player - so this doesn't sound like illegal substitute if he returns before the ball is snapped.