I don't know enough about the other two, but Bill Gates has been putting most of his money into charity for years and it's producing a lot of actual, measurable things. I don't think he's tax sheltering or scamming at all, and even if he was it's no proof of something happening now given that he's been at it for years.
As for Buffet's selling of IBM, that's just a good call. IBM hasn't been a great company for a while. I had some IBM stock for a year or two and it was by far my worst performer, and I suck at investing!
This is not just about those two but the general trend. Charities are the new tax havens since the panama paper debacle. there aren't many hiding places anymore. What most wealth managers advice (and i know a couple in Switzerland) is to shell through a charity and then trickle back up to other companies once the storm passes. even without a storm this is big tax protection.
Switzerland started the trend and in fact many blockchain companies are registered as such. I believe the latest Tezos sham had to do with the company being supported by a shell charity.
I didn't even mention the obvious warnings that are thrown in popular networks as a flag. and for a good reason.