I do agree, and I do think legally biding election could be a better way. Although,

1) Membership is not required to vote in WMF’s election. It is a requirement for Wiki Project Med’s election. Editors may want to vote while keeping their presence anonymous.
2) WMF and most non-profits are never structured to be a full direct democracy anyway, that may be ethically required for government elections. However, WMF is not a government even though those on Wikimedia projects may feel that way. Still, WMF is optional, not mandatory. It can set up rules as long as it helps promotes its mission.
3) WMF likely have paid a large sum to their legal counsels who suggested this structure as the best approach. Non-profit legislation is highly location-dependent. It also might just be how non-profit in California structure their election.

I’m leaning towards trusting the foundation on this matter without further investigation into the legal complexity of it. At least, until it reject one of the community elected trustees. I feel further effort would be expensive and likely result in no practical change...