Not everyone wants to play how you want them. Some tanks aren't comfortable switching stances, others simply like having the vit gear to make life easier on their healer.
Ultimately any extra dps from a tank is just that, extra and optional. Helpful.. yeah, mandatory.. hell no.

--edit-- add in that some tanks are just starting their expert runs/gearing up, and are hugely outgeared by dps there.. a whole lot of keep the fuck up with aggro. you lose it once, ya ain't gettin it back kinda thing.