I don't think it's appropriate, but I don't think it's out of bounds either. It's unusual to have that kind of open-door policy. However, what if something comes up that doesn't fall under the whistle-blower policy? Maybe they want to vent complaints about how the organization is being run and the work being done, not necessarily misconduct. If the Board of Directors knows what they are doing, they will listen to any complaints and react accordingly, realizing people may complain just to complain.
The thing is, is this just a policy as well, or is it done just via word of mouth? Can anyone do it or just select individuals? If it's select individuals via word of mouth, it's highly inappropriate. If it's everyone and there is an official notice on it, I would feel that's well within the bounds of what is appropriate.