Thanks, I emailed them:
# Solutions for the growing open source user base
"Hello, I am an AMD user, who uses Linux, and I currently use a small HDMI screen but I am looking to upgrade to a larger screen and I am concerned about the forum's decision to deny AMD's open source HDMI implementation. This decision has led to criticism of the entire HDMI enterprise as rent-seeking, and encourages non-trademarked implementations that weaken the HDMI brand. Linux is only growing in popularity, as people are tired of the advertising baked into Windows, and don't always want a Mac either, I highly suggest a re-think that looks at the rising popularity of open source with Linux, Android, Chrome and so on, to protect the HDMI brand.
They actually do at the bottom of [1].
[1] https://hdmiforum.org/about/