No group of people uses the SI system 100%, everyone just uses what is convenient (scientists use lightyears, and europeans use km/h, not m/s). The same is true of how americans use their system. There isn't an old unit convenient for small weights of this scale, even carat is too rough, so they use mg.

I don't know of any other examples of it being mixed quite like this, but I do see pressures given in "mm Hg" sometimes.

I usually hear metric for nutritional measurements and imperial for bulk food measurements so I am assuming that’s what’s going on