The Knitting Exception

I have noticed in virtually every meeting I attend now, people knitting. It strikes me as odd that if one were doodling, working on a crossword puzzle, or texting, that it would be seen as horribly impolite. Yet, there is an exception made for knitting. I've heard the excuse that I can listen and knit at the same time. But I can doodle and listen at the same time, but that would not be acceptable. While knitting, you have to be counting rows and keeping track of stitches, so it can't be true that it is truly independent of mental effort. Why is knitting the one acceptable form of multitasking?