I don't know what's evil, but I'll answer your last line.
StackExchange doesn't guarantee the right to fork: the software is proprietary. However, posts are free content, because they use a free license, the cc-by-sa (not just "a Creative Commons license"). Importantly, a data dump is available on Internet Archive and updated irregularly (at intervals of about 6 months). So, our posts mostly grow the digital commons rather than a private property.
The XML data dumps will qualify as open data if they are an open format:
which places no restrictions, monetary or otherwise, upon its use and can be fully processed with at least one free/libre/open-source software tool
There is some documentation on the database schema but I'm pretty sure StackExchange doesn't provide any free software to parse the dumps. However, according to the comparison of Q&A software, Askbot is (the only?) one free software which can import StackExchange data. Someone should verify whether it works.
For comparison (as you mentioned them) Wikimedia projects run entirely on free/libre software, MediaWiki and others, with open CVS and thousands of software contributors, and data dumps are provided every 2-4 weeks: the right to fork is guaranteed to a rather high level.
As for Wikimedia Foundation's ads, you can read their financial reports. For Stack Exchange Inc., see business model: there is some information on investors at https://crunchbase.com/organization/stack-exchange but valuation and revenue are secret.