Weird rule in NFL

I was just watching the Denver/Atlanta game recap and saw this happen and I remember seeing it happen in one other game this year and it makes no sense to me. 

Atlanta committed a penalty for illegal shift, but for some reason it's not blown dead, they complete a pass and the receiver then gets facemasked. The illegal shift penalty is disregarded and the offense gets a free 15 yds. If its not blown dead then that means its a free play for the defense not a free play for offense. So how does the offense get to benefit after their penalty? 

I dont understand how the illegal shift isn't blown dead like false start is. This game was a blowout so it didnt matter, but I'm pretty sure the other time I saw it happen this year it was a more impactful play. Regardless it makes no sense to me.

Illegals shifts are not penalties until the ball is hiked.

Up until that point, players can reset.

Illegal shifts only become a penalty at the hike of the ball.

@TJZags598

 Once they snap it and its a penalty though, its a free play for the defense. The offense cant gain yards unless the defense declines the penalty. So i dont think the defense should be penalized on their own free play. Maybe I'm crazy but it seems wacky

Keep in mind it works both way

If the defense just off sides, but the offense commits a 15 yard penalty ( unsportsmanlike like) the 5 yard penalty for offsides is wiped out.

Just as the offense can take a risk ( throw the ball long ) when the defense jumps offsides....The defense can now go for an interception, or sack, and take more risk.

The offense definitely has the advantage though

@TJZags598

 Here’s one for you TJ-

In the NFL, why does a personal foul that happens after the result of the play erase the play completely? For example a quarterback throws an interception and then gets roughed after the throw has been made. Why would the result of the play not stand and the penalty apply to the next offensive possession? After all, if a QB throws an interception, nothing that happens after the fact changes the fact that the QB threw the interception.