This slippery slope goes in both directions. Software is also easier to audit when it's built in higher-level languages, but we don't have ideological objections about verifiability to software written in C.

I'm not sure I understand what you mean by that. Do you mean that people think that it doesn't matter what language code is written in as long as it is open source? I certainly don't believe that. It's pretty clear to me that C is a terrible language for writing secure code. (But coming up with something that is actually better than C is not so easy.)

All agree that higher level (e.g. C) is easier to reason about than lower level (assembly).

Now, you say that open source (e.g. C) is not only easier, but qualitatively different: open source good, closed source bad.

tptacek points out: higher level (e.g. Haskell) is easier to reason about than lower level (e.g. C) - maybe even qualitatively.

So, why are people only complaining about closed source, when they should (by analogous reasoning) be complaining about code written in C? Granted, it's possible to analyse, but it's obviously easier when it's written in Haskell!

For what it's worth, I complain about code written in C too.