Sawdust is not (just) cellulose and cannot be listed as such on nutrition labels. Sawdust, i.e., wood shavings, contains many other compounds, especially lignin. Wood is refined by e.g. the Kraft process to separate the lignin from the cellulose, giving a suspension of cellulose fibers in water called "wood pulp." I didn't look, but I would imagine that calling wood pulp "cellulose" on a nutrition label is fine, 'cause that's what it is.

Now, none of this invalidates the crux of your argument that cellulose can be used as a cheap filler, such as in cheap "Parmesan cheese," and no disagreement here that that shit is scummy af. However, there are some legitimate uses for smaller amounts in foods, such as anti-caking, thickening, and literal dietary fiber.