This one has a semi-complicated answer. You're correct in that being bilingual is typically a choice, but language imperialism (or imposition) can contribute to being forced to become bilingual. Let's use your example of Hawaii.
When Hawaii was colonized by white settlers, they brought English with them. They forced the Native Hawaiins to learn English because that was the "civilized" language. In reality, what they were doing was erasing part of Hawaii'n culture and identity. Linguistic imperialism is when a dominant people forces their language upon another people. In the case of African slaves brought to the United States during colonization (and beyond), this can lead to their languages being completely erased as they are forced to speak English and forbidden from speaking their native languages.
The term "bilingual" just means one can speak two languages. Typically bilingualism means the speaker retains the two languages (someone who learns to speak English as a second language, for example). Linguistic imperialism differs in that it often causes the complete erasure of one language in favor of the dominant language.
I hope this helps!