Old ATI firmware is freely licensed (under an MIT-style license like the driver), while there is no source code nor documentation available. We don't know what instructions there are nor what it exactly does, so we cannot study nor modify it as easily as AMD can. New firmware (some for R600, all for Evergreen and newer GPUs) additionally has a nonfree license.
I think "legal freedom" doesn't make it easier to understand. Both technical (lack of source code, lack of documentation, cryptographic restrictions) and legal (copyright, anticircumvention laws, patents, plain censorship) ways are used to restrict user's freedom and give power over them to the developer. A different issue started the thread: software being free doesn't mean that it has all possible features.
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