Take a look at the original json.org license and all the problems that the "not for evil" clause they added to it had caused.
Ultimately though, if you put a non free license on your libraries, somebody will cry foul, fork it, and evil will still happen.
Some background in https://gist.github.com/kemitchell/fdc179d60dc88f0c9b76e5d38...
Basically you end up with something not legally enforceable. And will someone actually doing evil care about your license?
Well then you just use some copyleft language to ensure the same license (or something you deem compatible) is used.
Just because you can fork something doesn't always mean you're able to just change the license.
I suspect the non-standard JSON license was in part a strategy to encourage third-party implementations, so that the format would become a standard.
(W3C standards, for example, require "multiple independent implementations to proceed along a standardisation path". https://www.w3.org/TR/webdatabase/ )