(not a lawyer but I have a decent grasp of how this works in common law countries)
Board members are required to always vote in the best interests of the organisation they are on the board of. More specifically, for nonprofits, they are required to vote in the best interests of the mission of the organization. This is because a nonprofit has no interests except the pursuit of the goal that the nonprofit has.
There is no real definition of what 'best interests' represents. Board members have very wide discretion and judgement about how to interpret that. The income, assets, staffing etc of the organization all exist towards the organization's goal - and while there is a presumption that having money, assets, staff etc is a good thing, boards are perfectly free to make choices that result in having less money/assets/staff (or indeed to e.g. merge or wind up the whole organisation) if they see reasons that will be better for the organization's mission.
It's quite legitimate to have a Board member who views the interests of the WMF and 'the community' as inseparable. Just so long as the person concerned has arrived at that conclusion themselves and not due to undue influence from somewhere, or conflicting financial interests, or similar. However, if a Board member perceives that 'the community' and the WMFs mission are in conflict, they must prioritise fulfilling the WMF's mission.
To apply this to the Movement Charter situation, there are 2 ways the Board could fail to comply with their duties. [Just to be clear, I think they are complying with their duties fine, even if I disagree with the decision they end up making, these are just illustrative examples]
1) Board members could look at the MCDC's draft charter and think they are obliged to say yes because the MCDC drafted it. This would be failing in the Board's duty to apply independent judgement in the best interests of the WMF's mission.
2) Board members could look at the draft charter and think that, because it costs money the WMF could spend elsewhere and potentially restricts the WMF's own scope, they are barred from saying yes to it. This would be failing to look at the whole context of the WMF's ability to fulfull its mission.
Essentially the Board members have to look at the facts and the context and make a decision about what they think is the best path to achieving the WMF's mission. So long as they do that, they are behaving correctly and legally as Board members.
Regards,
Chris