Most people use their computers to run a web browser, maybe a word processor or media player, and... not much else. Even someone who has only used Windows can figure out those basics on a Linux desktop.
If the charities are unable/unwilling to provide support for Linux, they could give computers away on Craigslist before dumping more e-waste into our environment.
I did that for my grandmother with FreeBSD many moons ago, on a Pentium3 no less. It ran for years and years like a champ. Booted straight into PySol since that was pretty much all she ever did on a computer.