simple showdown question

[RR]

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HU pot. Player A bets out. Player B calls. Player A throws his cards in the muck (on purpose, just surrendering his hand). (Edit: this is on the river if that wasn't clear)

Why does player B have to show his cards to get the pot? Is this just to prevent collusion?

Is this the "standard" showdown rule?

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It is not standard. Even in those rooms that require him to show it is not clear what they intend to do if he doesn't show.