I'm not shader, but I'm guessing that it's because... you can't.... bind values to strings...?
I mean, yeah, you could have a table where the keys are strings, and treat that as the metadata, but uuugh it's clunky and I've found myself disliking the whole "global table to hold data" thing more and more, recently.
Then again, I don't know how their compiler works, so I might be completely off-base here...
