I think you're right. The Hebrew sentence (probably) refers to the tomatoes as healthy not the act of buying them, so it should be "they are healthy".
No, "they are healthy" would be "הן בריאות". In both the Hebrew and English, it is inferred that "it/זה" refers to eating tomatoes.
I don't think English and Hebrew are the same in this situation. Compare "עגבניות זה בריא", a legitimate sentence, with "tomatoes is healthy". In Hebrew "זה" is used for this kind of general statements - "ספרים זה סטימצקי", or "צעצועים זה אנחנו", or "מים זה כל מה שאני צריך" (all of which are plural), "אכילה מרובה זה מה שגורם להשמנה" (feminine) etc.
So in this case, in the Hebrew sentence, there's no need to infer anything - the meaning is simply that "tomatoes are healthy". In the suggested English translation, either you infer "eating" or you understand incorrectly that "buying tomatoes is healthy". But either way, "Buy tomatoes, they are healthy." is the best translation in my view.
On second thought, you may be right. I'm not sure about your examples though, because they all use זה as a copula (אוגד) while in the sentence above it is the subject.
The first clause and the second clause are connected by "it". So "buying tomatoes, it is healthy" connects "it" to "buying". And "buy tomatoes, they are healthy" connects "they" to "tomatoes". But "buy tomatoes, it is healthy" leaves unclear what "it" refers to, so it is a poorly constructed sentence. This is an example of a dangling participle.
I recognize that English speakers commonly (mis)use "healthy" in this way, but the correct word is "healthful". Duolingo marked "healthful" as incorrect.
"...it's healthy" is certainly not good English, if we're meant to understand (logically, it seems to me) that eating tomatoes is healthy.