This is the sticking point. You've basically admitted that the problem is the possible appearance that we're "promoting" unfree software. Not that we're actually depending on or delivering unfree software. 

The idea that we're somehow widely and officially promoting unfree software here is frankly a gut reaction that is not supported in fact. Users will need to inspect our CSS in order to even view the font settings. Most users do not know how to do this. For those that do (i.e. programmers), they should know well enough that CSS means we are not delivering un-free software, but rather doing what almost every site without webfonts does. That is: listing a font stack that is appropriate for users of many platforms, free and unfree, mobile and desktop.  

The vasty majority of our users are on unfree platforms and/or do not have quality FOSS fonts at their disposal. Our CSS and other styles should look readable and beautiful on those platforms. The good way to do that today, given our constraints, is to explicitly acknowledge some small parts of this in our font stack, while also making it as widely usable across platforms. CSS and browser font styles are really good at doing this, with fallbacks etc. 

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