Sorry if there's a bug already for this. We recognise that AMO hasn't got the support for static themes yet, but the support dynamic theming is really awesome and pretty powerful. Unfortunately the two are tied together behind one pref and it would take some time to detangle them. The suggestion, which got a general round of approval was to turn this pref on by default in nightly so people can use dynamic theming. Static themes will get an error if you try to upload them to AMO at this time. Adding in dev-doc-needed because I'm not sure where documentation is at this point on themes and also to point out the static / dynamic difference in support.
Although bug 1341722 just mentions Nightly and Dev Edition. I think it should be on by default to release.