> Sentences can't be extended without additional convictions.

This is technically true but substantively false. Fixed duration sentences in most US jurisdictions (life sentences are different) are come with essentially automatic substantial reductions for good behavior which are removable for poor behavior with minimal process, avoiding the hassle of judicial process for offenses in prison, and frequently “refusing work” is a cause for removing those reductions.

So, technically, its not an “increased sentence” for refusing work. But, in practice, that’s exactly how it functions.