This is a followup to Why are CS researchers reluctant to share code and what techniques can I use to encourage sharing?. That question specifically asked how one can succeed in obtaining researcher's source code.
As discussed in the answers to that question, the reasons largely boil down to competitive advantage and people thinking their code is not good enough. The former issue is difficult to address. However, one could try to address the latter issue, making the reasonable assumption that this behavior stems from the surrounding academic culture. There may be additional aspects of the academic culture that discourage code sharing, and which do not relate to competitive advantage.
So, one could instead ask the general questions what concrete actions one can take to change this culture? Or, to put it a little differently, how can I help change the academic world so that more researchers are willing to share their source code?