I agree it shouldn't be, and maybe everyone who was taught standard acol was taught to use 4NT as quantitative at the appropriate point, but I think lots of beginners may be taught blackwood and not be taught (or not remember) a quantitative 4NT, in which case I expect an even split between "oh 4NT, must be blackwood" and "no, wait, can't be Blackwood because we're already in NT, not sure". I think I've seen or experienced that, although I'm not sure.