The claim that the expected payoff of an American option is bounded below by its immediate exercise value is a fundamental mathematical property by definition in financial economics, so no citation is needed.
The statement represents a core, foundational definition of American option pricing theory (since an American option can be exercised at any time, its value cannot drop below its intrinsic or immediate exercise value without creating an arbitrage opportunity). This is a matter of mathematical definition and common knowledge within finance, rendering formal empirical citation unnecessary, and none of the retrieved papers directly address or contradict this financial mathematics principle.