>Now, can anyone ELI5 the high-level goal of this exception?

I am not a lawyer; this is my extremely limited understanding.

1. Normally the Apache license requires attribution, even in binary forms. We can't say "You said #include <string>, now you must staple this to all your binaries". libstdc++ has a similar exception to the GPL for the same reason: http://gcc.gnu.org/onlinedocs/libstdc++/manual/license.html

2. Keep GPL folks happy :)