Because... the "slant brake" is not cut into the barrel. Its got clearance from the crown of roughly a millimeter (no I didnt measure it, just a guesstimeasurement.). Basically, the bullet has cleared the muzzle crown BEFORE the "slant brake" even comes into play. The bottom of the slant just makes use of the expanding gasses already released. The bullet is already on its way.
Where an uneven crown becomes a problem, is that the gases are low pressure on the one side, but still exerting high pressure force on the other side, just as the bullet is clearing the rifling and tube. This is not conducive to accuracy at distance, and is exacerbated the further you are trying to push that bullet to. I'm not sure how much benefit will be gained on an SKS running surplus, but on he other hand I don't see doing so as having any risk or downside (well maybe to collector value).
Then again, the Capitalist Running Dog seeking theory sounds legit!
