。
A polite American is polite at all times.
Literal
Polite American [topic-は] always polite [copula-だ].
A generalization. The relative-clause construction 礼儀正しいアメリカ人 ('polite Americans') modifies the noun with the bare adjective, no の or な needed (since い-adjectives modify nouns directly). いつでも ('always, at any time') is adverbial.