This from the case book...
9.6.1 SITUATION A: With third down and 20 from B’s 40-yard line, A1 accidentally
steps out of bounds at B’s 30 while running a sideline pattern. A1 returns
inbounds at the 28. Quarterback A2’s forward pass is: (a) overthrown and incomplete;
or (b) caught by A1 at B’s 25; or (c) in flight to A1 when B1 contacts him
and it falls incomplete. RULING: In (a) and (b), it is illegal participation by A1. In
(c), A1’s illegal participation and B1’s interference result in a double foul and
replay of the down. COMMENT: When A1 goes out of bounds voluntarily or by
accident, or delays his return after being blocked out by an opponent, he commits
an illegal participation foul when he returns inbounds. The spot of the foul
is where he returns inbounds. (10-2-1)
I'll stand corrected on this but I still believe it should not be called IP. This is similar in nature to the holding call away from the point of attack, or when the opponent does not fight to get separation. We have holding, but we don't have a foul. We've set the bar high for holding calls: big, at the POA and involves restriction.
Since this is a simple "right foot out, left foot in" case, I would simply call the pass incomplete with receiver OOB. If there was a noticeable delay in returning, or he went further OOB than just a foot on or slightly over the sideline, I would flag for IP. Use good judgement here. That's a big penalty for stepping on the line trying to catch a pass while running without looking where you're going.
If the player still has a foot on the line, sure, call it incomplete. It is, no problem.
But the problem comes here: A runs near the sideline and without being forced, clearly steps on the line. He then cuts back to the numbers and catches a pass 12 yards downfield. Now what? You can't call it incomplete, he's nine yards inbounds!
Do you ignore the fact he stepped on the line because it was an "accident"? Sorry, that's poor officiating. You KNOW he stepped OOB and came back.
Many fouls aren't advantage/disadvantage. Defense jumps into the neutral zone before the snap, makes no contact, no one on the offense moves, and the B player is retreating when the ball is snapped. If anything, the defense is DISADVANTAGED because the player's momentum is now moving backward. But it doesn't matter, it's a foul in FED.
A lines up with 6 players on the line, and 4 in the backfield. What advantage do they gain? None, they are actually DISadvantaged because they only have 10 players on the field. But it doesn't matter, it's a foul in FED rules.
I don't like the IP rule in this case. It doesn't differentiate between the player that accidently steps on the line from the one that runs OOB, stays out for 5 yards, and comes back behind some teammates downfield. Both of these are 15 yard penalties, and that doesn't seem right. I would be in favor of a rule change that simply makes a player that ACCIDENTLY steps OOB ineligible to catch a pass, making the pass incomplete.
But that's not the rules, and calling it that way isn't "good judgment", it's making up rules.