I'm no lawyer, but the way I read that decision is: do what you think best, just don't purposefully mine the field against one of your shareholders.
The practical question is not necessarily whether Craigslist has a duty to maximize shareholder value, but rather, to what extent can shareholders use the courts to second-guess the business decisions of corporate management? As the links in the Google search above attest, decades of precedent seems to suggest: almost no extent at all.
To look at another high-profile example, Apple CEO Tim Cook--in a shareholders meeting!--said "When we work on making our devices accessible by the blind, I don't consider the bloody ROI. If you want me to do things only for ROI reasons, you should get out of this stock." While it made some news, I don't recall a shareholder lawsuit forcing Tim out of Apple for refusing to maximize shareholder value.