Dr. said:
If someone has a clear understanding of the knowledge/belief distinction, the “agnosticism” label is semantically valid. But as you say, many use the term to indicate uncertainty - or a diplomatic/non-confrontational middle ground - between atheism and theism. And I think many religious folks tend to respect “agnosticism” more because it connotes open-mindedness (hey, maybe there’s a chance the agnostic will see the light). They’d be far less sanguine if they understood agnosticism to mean “positively unknowable.” Much of the tolerance towards true agnosticism isn't really earned - it has to do with the ambiguity of the term.
For most other categories of non-belief (e.g., unicorns, leprechauns, ghosts) we have no difficulty being definite - even if, technically, we should be agnostic. But saying we’re agnostic would suggest an equivocation that doesn’t properly reflect our extreme skepticism. Yet when it comes to the god hypothesis, plain communication seems to go out the window. Suddenly, people are lawyers.