Now, the FTC is taking action, warning companies (including Amazon, Target, and Macy's) that they must change their labeling of "bamboo" fabrics to "rayon." While the FTC does not seem to have targeted yarn manufacturers in their efforts, presumably the same implications would apply. If your yarn label says bamboo, it probably means "rayon." The same goes, presumably, for yarns made out of corn or hemp.
The FTC has a useful guide here, if you want to know more. Interestingly, the clothing & textile regulations do not seem to apply to yarn--from what I can find.