Actually he wasn't.
The rule is you cannot have pass interference behind the LOS, that's what you are arguing I'm sure and I understand that, but you'd be incorrect in assuming that he was behind the LOS.
You'll notice here that the LOS is essentially the 51 yards line.
Fowler runs passed the 51 yard line, gets tackled by the WR and ends up behind the LOS. This was the moment of impact.
It was really close, but it was past the line of scrimmage and as such, tackling a WR past the line of scrimmage is a pass interference penalty. The ref most likely saw this upon review of the tape, but since it's not challengable, could not do anything about it. All he could do is review the turnover which did not touch the ground and therefore was a turnover.
My guess is after reviewing the play (because all turnovers are reviewed by default) he came over and explained that "hey, we missed the call, but since we cannot review it, the turnover has to stand because everything that was reviewable was valid."