It's amazing how so many people feel entitled to an opinion about what someone else does with their own money.
The way I see it, there are only two people in this world who can possibly have the right to say anything about what Mark and Priscilla do with their money and those two people are Mark and Priscilla.
Whether they're doing it to save on taxes or change the world is really not the point. It is their money, not ours.
If you want to be entitled to an opinion about what to do with ±45 billion USD, you should get off your behind and earn some money yourself, then you'll have the right to decide what to do with it.
For argument's sake, let's imagine for a minute you had a big stockpile of cash and you wanted to make a positive impact on the world and perhaps leave a legacy behind. Would you blindly give it to some charity, with potentially bad governance and highly paid employees, making little real impact on the world, would you give it to a corrupt government to squander or would you want to have some say in what happens with your money and try to influence the way it is used for positive good?
Whatever your choice, if somehow it resulted in you saving taxes along the way would you consider that a bad thing?
While on the topic of taxes, some people keep complaining about how other people and companies set up complex structures to optimise their assets and reduce their tax liability. Let's make something very clear, these structures are expensive and the obvious part, complex.
If you want to set up a structure to manage your money that is complex and expensive to operate, there is no law prohibiting you from doing so. You are free to hire expensive lawyers and accountants and pay all the fees required in order to establish and operate any structure you desire but chances are you won't bother to do it, chances are you won't even research the options, chances are you won't even bother to change your savings habits to reduce your taxes by making medical contributions or saving for your own retirement.
You see the thing is, a good portion of earth's human population is not interested in adding value to the world, the majority are too lazy to even do anything to improve their own lives and would much rather spend their spare time watching sport or soapies than improving themselves or building a profitable business. They see their weakness and failure in the success of others and their default reaction is jealously instead of motivation to be better and do better.