This is actually not the case.
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operations.nfl.com
A feet first slide is given additional protection under the rules. Any contact with the head or neck in a feet first slide is a penalty, regardless of timing. Not the case with a head first dive. E.g. you can put a shoulder into a QB trying to launching head first across the line to gain in a qb sneak.
There are 11 ways in which a player is considered defenseless.
I think the best argument for the penalty is that he was "on the ground" or that the defensive player dived into him.
The problem comes with reconciling Rule 12, Section 2, Article 9(b)(1)
with Rule 7, Section 2, Article 1(d)(2)(3):
and Rule 12, Section 2, Article 8(e)
Rule 7, Section 2 talks about when a player is down and the ball is dead. It's not clear what "this protection" means. Does it mean Rule 12?
Rule 12, Section 2, Article 9 is about defenseless players and just requires that a player is "on the ground." It's not clear exactly what on the ground means (a player on his hands and feet is technically "on the ground" but not down).
Rule 12, Section 2, Article 8 talks about unnecessary roughness. Does "throwing" your body at someone require you to leave your feet?
I think they got it wrong, but it's definitely a judgment call and not cut and dry.