Biologists tend to treat gene expression as functional. In E. coli, this is much more likely to be true than for humans. First, E. coli is a single-cell organism so there is not conflict in differential regulation across cell types. Second, E. coli has a large population relative to its information content, which allows natural selection to operate more efficiently.
mRNA transcription (i.e., gene expression) by itself is relatively inexpensive, so natural selection only weakly optimizes expression in multicellular organisms with small populations. Protein production is much more expensive and is under stronger selection.
This. I love the honesty about the scientific process. This is completely lost to most science students while they cram facts before an exam. The road to most beautiful discoveries is often paved with lots of fumbles.