Because it's not.
Here's what the rule says again:
You have to take things with these rules at face value. The first four words are "WHEN THE BALL IS SNAPPED." The ball is NOT being snapped when he's moving forward in the snip below. Also this paragraph applies to players in the backfield, which of course he falls under.
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This is the precise moment the ball is snapped. When you watch the video he's not moving forward. Maybe a bit to his right because of his momentum, but not forward.
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Here's another angle of it:
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He's NOT moving forward at the time of the snap. Watch the video again, he comes up, stops for a moment, and then runs his route the moment the ball is snapped. And the rule does not require him to stop for a full second in this situation, as long as he's not moving forward.
Next paragraph in this rule is this:
This is where reset comes in, but it's for a player on the LINE. Rob Gronkowski is playing a "back" position in the backfield. So this doesn't apply in this case.
And the last paragraph states...
Rob Gronkowski is not under or behind center.
His movement doesn't fit into any of these situations, and that is why there was no flag. Not because the refs "allowed them to get away with it."