If the turtle-saving foundation I donate to has a windfall, I would expect it to save more turtles, not to instead also focus on feeding hotdogs to hungry Somali children. This does not mean that feeding hotdogs to hungry Somali children is wrong, it is an even more important goal than saving the turtles, but somewhere in the process, there pillaged a decision maker that misinterpreted (acceptable) or ignored (unacceptable) the intentions of the benefactors.
Except the foundations mission statement isn't turtle-saving, that's just what you think is clearly the most important part of their work that they should solely focus on, in your opinion. And it's not wrong of you to want to be able to donate towards the parts that you care about, but it's an obvious disconnect from the reality of what their stated goals are (and hence you blame decision makers for decisions that are actually fully in line with the mission statements).
Mozilla Foundation never was the Firefox Development Foundation, as much as some of us want it to be. Wikimedia never was the Wikipedia Development and Operations Foundation. If you dislike that, donate to entities with narrower goals (I personally prefer directing money towards OSM and KDE for example)
If a foundation spins off a project's community with certain goals, they take on to some degree the goals and the vision of the people that built it. Those are opinions, and they are important to the cited goals. If then a foreign element enters the system due to the sad state of the technology/corporate enterprises nowadays and changes the mission statement, you have not changed the spirit it was created with, you changed a business strategy in a way that is disconnected and hostile to the initial goal. A lot of the times the goals aren't really stated at all clearly, are protected by different acts like codes of conduct, and open to modification by impassionate people with ephemeral leadership positions who don't care what happens to the organization down the line.
Sure, I can become the chairman of Turtle saving international and change our charter to prioritize feeding hotdogs to hungry Somalis, but I am still doing an ideological disservice to the grassroots initiative that built the foundation and created the position for me to be sitting on, no?
Also, this seems to me extremely fragile when situations are reversed, say that an outright awful organization like an international petrol company gets a new mission statement. Should all they did before and after that be forgiven because their stated goals say otherwise?
I personally see this as little less than a ground truth, but perhaps there is some way that stated goals stand above all regardless.
EDIT: I do see how you make a very good point when you're looking mostly from the present towards the future, though.
In both cases, their "wide" mission statements have been around for a long time, it's not a recent thing. E.g. you maybe can say that Mozilla Foundation in 2003 was presented as something else than what it decided to adopt in 2007 with the Mozilla Manifesto, but well, that's a 4 year period that ended 18 years ago and happened under the original leadership, it's not later executives somehow distorting it.
