Greenberg assumes that one of the characteristics of unmarked categories (in this case the singular) is that in many instances they stand for the category as a whole, and hence they may be used with the plural, while the marked category of the plural is restricted to actual plurality. Whenever this holds, the cardinal numbers >1 are included in the situations in which a singular form of the noun may, or usually must be used. See also #549.
Greenberg assumes that one of the characteristics of unmarked categories (in this case the singular) is that in many instances they stand for the category as a whole, and hence they may be used with the plural, while the marked category of the plural is restricted to actual plurality. Whenever this holds, the cardinal numbers >1 are included in the situations in which a singular form of the noun may, or usually must be used. See also #549.