Why can't people hear past the upspeak?

I've written about upspeak before, but the comments on this Language Log post surprise me (not so much the commenters themselves, but what they are saying about how people in general perceive upspeak). It sounds like some people can't hear beyond the upspeak. They hear upspeak and think "ditz" or "insecure", seemingly without giving a moment's attention to the words being used or the ideas being communicated.

That seems utterly bizarre to me. Why should my intonation blind my interlocutor to the fact that I'm using the words "intonation" and "interlocutor" rather than "how I'm talking" and "the person I'm talking to"? If, instead of "Things They Should Invent", I titled my blog posts with "I wonder if this exists?", why should that affect the perceived merit of my ideas (insofar as my ideas might have merit). Even if upspeak was a sign of insecurity, a useful idea expressed insecurely is still useful. If the solution to all our problems is a red widget and I say "Um, I kinda have an idea? Just putting this out there, I don't know if it's any good, but what if we got a red widget?" how could my uncertainty stop everyone else from immediately making the mental connection that yes, a red widget will solve all our problems?

When someone is talking in your first language, you hear their words and understand the content without any effort unless what they are saying is way too difficult for you. If someone is speaking with a low-prestige accent and clearly communicating good ideas, you still automatically hear the words and understand the ideas and can quickly grok that they know what they're talking about despite their low-prestige accent. It takes no effort to do this in your first language, your brain processes it automatically.

So why doesn't it do the same thing with upspeak?