The problem with "letting them play" is that the players very quickly realize that they're getting away with things they shouldn't and the offending actions start escalating - the line of what's legal gets pushed back - the games get very chippy, and then usually end up one of two ways - getting decided on a penalty for something that's been 'ok' for the last 25 minutes, or with players taking swings at each other (like the end of the superbowl last year).Said differently, if the refs indeed affirmatively decide to let them play in the SB, why isn't that standard applied to other games? I never understood that, nor have I ever understood the swallow the whistles thing in OT hockey, to the extent it still applies.
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The Pat Chung holding call was a good example of this - it was a pretty clear hold and should have been called - the problem is that the refs mostly swallowed their whistles and let defenders play - and that had been "ok" for most of the game - he just did it in the wrong place, or too visible to a referee - and then they went back to what they were doing before and basically let Talib tackle a crossing WR in OT. Not enforcing the rules doesn't work - the players will force you to enforce something at some point - and it's going to be inconsistent.
On the other hand, in some games you'll see 3 or 4 calls early in the game, and the rest of the game will be pretty clean - the players quickly figure out what the constraints are for the environment they're working in.