Why does the sentence not translate "this is business" or "it is business?"
It's not a sentence, it's a nominal phrase (that's why it doesn't have a capital letter).
"This/is is business" would be «Э́то би́знес». «Э́то де́ло», when used as a sentence, has an idiomatic meaning 'it is a bargain, I agree to this', so it's unlikely to be used to mean 'This/it is business'.
ooo, that's sneaky, gotta admit, I don't pay much attention to whether the word is capitalized or not. Thanks for the heads up!
I was under the impression это was a a pronoun and этот was a determinant. Is this wrong?
Это is a not just a free-standing pronoun, but also a neuter-gender form of э́тот. So, «это дело» can mean both 'it/this is [a] business' and 'this business'.
Even when you check the word here on Duolingo it suggests "case" as a translation.
дела is the plural form of дело. They differ in pronounciation because they have different stress patterns: дело/дела
Oxford dictionary gives very many meanings, and these should all be included. There is no context given here, with a two word phrase, that would exclude any other meaning. Also, the Oxford dictionary gives synonyms including работа, and занятие. The various meanings are business, affair, deal, cause, occupation, concern, matter, point (as in point of honor), fact, deed, act, case. There are other meanings, but that case and work are refused is something with which I would take issue.