Learn English – Pronunciation of en- prefix as ahn-

For words which are commonly used in English (like "Paris") using the foreign pronunciation is guaranteed to sound pretentious.

English has a distinctive phonetic pattern, and switching to another phonetic system in the middle of a sentence for the purpose of pronouncing a different place name would sound odd and draw attention to yourself. Imagine an American speaker switching to a British accent every time he had to say "England". When speaking in an American accent, saying frahns instead of fraens (for France) is unbearably pretentious... even though frahns is the way it's pronounced in British English.

For more rarely used words (for example, small place names rarely mentioned in English), using foreign pronunciations won't be noticed. Nobody in America has ever heard of Ramat Gan, Israel, so pronouncing it the Israeli way (raMAT GAN) instead of the English way (RAMat GAN) won't even be noticed... but with American audiences, switching to American phonetics is always appreciated (using a hard American R instead of a gutteral or fricative will help people understand what you said).

Apart from this related answer, Etymonline has:

buoy (n.) late 13c., perhaps from either O.Fr. buie or M.Du. boeye, both from W.Gmc. **baukn* "beacon" (cf. O.H.G. bouhhan, O.Fris. baken). OED, however, supports M.Du. boeie, or O.Fr. boie "fetter, chain" (see boy), "because of its being fettered to a spot."

So you have two possible origins, one originally pronounced [bɥi(ə)] (French) or [bœɛi] (Dutch), and the other [boi] (French) or [bœi] (Dutch), all of which could be Anglicised as either disyllabic [buwiː] (boo-ee) or monosyllabic [bɔɪ] (boy).

I suspect both pronunciations have been around for a while in English, and the colonial divide just drew a more distinct (regional) line between them.