To native British English speaker, the first sentence parses easily and has a good rhythm, but the version with "flowers and orchids" is confusing and difficult to parse:
If spoken, there would need to be a pause before the first "and", otherwise the listener hears "she loves flowers and orchids", just before s/he hears the unexpected verb "are".
If written, one could place parenthetical commas to make the sentence work: "Because she loves flowers, and orchids are flowers, [therefore] she loves orchids." but that version still sounds ugly to me.
However, once the second sentence has commas, I think most people would consider it a matter of style. I had a quick look in Fowler (a dull and overly prescriptive British style manual) but didn't find anything relevant.