To me the interpretation of Mercury (Hg) is the weird thing, and I guess they accidentally 'classified' 'MicroMeter" as ?m of Hg.
But I agree with all the posts above that Feet, foot, feet, Foot et cetera should all work.
In general it should not be case sensitive, because I don't think you can confuse two units by their case.
What I still don't get; if these queries get handled by Wolfram|Alpha, why are we not getting this piece of script in Mathematica? Why send these queries over the internet, unnecessarily waste their server-resources, and give the end-users (us) a slow experience (that requires internet connection)?
Can one of the developers please explain why this decision has been made?