That interpretation makes total sense given the statements alone, which is probably what makes it a good example. Here I think you must pay attention to the order statements are in and how they are stringed/attached together. The premise “Since none of them have devised a suitable recycling or disposal plan” is joined to "they should stop producing food waste and shut down operations immediately" with a comma, indicating that the relevant support relationship exists between the two. If the example instead read "But this is not a sustainable, long term solution, since none of them have devised a suitable recycling or disposal plan." with the conclusion standing alone, I believe you would be correct because the punctuation in that case indicates a different relationship between the statements. (I guess you could think of it like order of operations in math. Punctuation acts like parenthesis. It indicates the order in which operations are solved.) The TLDR: Punctuation matters. It indicates what statement the premise supports.