Special:UploadWizard, subclasses Special:Upload. It then reimplements the entire execute, part of which is to include a 'dumb' special:upload uploadform. The execute however doesn't reimplement most of the functionality that Special:Upload provides and this includes loading the defaults for that upload form. Specifically, it doesn't run loadRequest(), which parses all of the request parameters that Special:Upload can provide. As a SIDE-EFFECT of that parameter parsing, Special:Upload also sets the default for the mComment variable (because it depends on the value of mReUploadFor). This brings up several interesting questions:

1. Why does 'loadRequest' in special upload set the default ? Move it out of loadRequest ?
2. Do we copy the loading of this default into Special:UploadWizard::execute() ?
3. Should we just render a page with a link to Special:Upload and a message of "This page requires javascript" instead ? That seems much safer. We probably won't even need to be a subclass of Special:Upload any longer.