Purposely scratching in 8-ball?

[Quadstriker]

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Bar rules vary from bar to bar. They often aren't written down, but the regulars know them well.

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God forbid having a written down standarized set of rules for a game, rather than some mystical "bar rules" that not everyone knows, changes at will, and very few people can logically explain. Can anyone give a good reason for this other than wanting to start fights with strangers over unknown technicalities?