I don't think that's fair at all. first off, the executive chairman of a company is usually responsible for heading up the board. they are not involved in day to day activities - it's not up to him to agree or disagree with how the company is being run day to day, but to determine if the company is on the right trajectory.
an executive chairman, or chairman of the board of directors, who ousts a new CEO without allowing the new CEO time to develop their strategy is not a very good executive chairman. future CEOs will be very wary of such a person - they will have no faith that this chairman will allow them time to settle into a tough, complex role.
second, and most important, the executive chairman doesn't have the ability to appoint or oust people independently. it depends on corporate governance policies, but usually - either the entire board has to come to a consensus (if it's not during a board meeting), OR there needs to be a majority consensus (during a board meeting / during an in person meeting) to oust the CEO.
reddit's board is not just ellen pao & alexis ohanion. sam altman is on there (he was part of the decision making process when yishan left). I don't know if there are others, but usually boards will have board observers and advisors who don't have voting capacity but are still very influential.
as to why these people are not up in arms re ellen pao. it really depends. sometimes, it's just a matter of the CEO wielding a lot of power with the board by being chums with a good portion of the board of directors / board observers. if she's stacked the board with a bunch of people who are her friends, then it would be a long term dangerous move for altman and ohanion to try and oust her. maybe she's friends with altman. maybe she & alexis are friends.
point being - I think this situation is the responsibility of ellen pao, first and foremost. as the CEO of the company, responsible for the day to day operations AS WELL AS the general vision & direction of the company, she needed to handle this better.
second, it is also the board's fault. nothing in her background indicates that she has any capacity to run a site like reddit and yet they allowed her to be the interim CEO. why not try & find someone who has experience running a community driven site? who understands the nuances of having a really massive, opinionated community?