Those are cogent issues with this type of brainteaser. However, it is a common assumption for the brainteaser that all subjects are intelligent and rational.

The idea isn't to find the real-world solution, but to find the correct solution (for some value of correct).

I guess the way I think about it is that a chess GM can say "Mate in 8 moves." Ok, that's great. However, it's mate in 3 moves unless your opponent does the perfectly correct move. The GM has to assume perfect play when making that assertion, not normal play.

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.