Public repos on github are free, private ones do cost something, i believe you get/got 5 private repos for 5$ a month (i have 2 unused private repos left, so no problem with using those, in case i'm trustworthy )
The 100$ is for companies (i don't even know the befit of having a company account)
I believe GitHub's public repos requite the code to be released under the GPL license. Which Alblaka may not wish to do. Also GitHub isn't a silver bullet. You still need trusted people to review and monitor the incoming code to make sure it is up to standard and does not do anything bad. The time spent doing this, appointing people to do it and managing those people is far from small.
Making the source public isn't a bad idea. Providing there are clear and strict rules about what you can do with it (I propose we use the same rules we use now, ie we cannot upload modified classes to anywhere but the addons forum, etc). I have seen a piece of Minecraft related code where the author shared the code, and wrote a license next to the download link about what you can and can't do with it (the code was basically for learning only). Needless to say the entire thread was turned into a fight between the author and another person, who took the code and made a ripped off version of the plugin. The other person claimed that as soon as code was released it was open source under the GPL license (which is far from true).