Between different manufacturers there is easily an 10x difference in price of 7805, so counterfeiting is certainly worthwhile, even more so when 7805 is more of a description of function than of implementation. 7805 means three-pin linear regulator with dropout <= 2V, such and such accuracy, line/load regulation and noise and typical design uses these parameters, not actual parameters guaranteed by given name-brand manufacturer (which are sometimes significantly better than for original LM7805), so mostly no-one will notice if you take random Chinese 7805 and repackage it into name-brand package.
