I know what's behind technically, with that question I'd rather meant what's
the big deal for an OS
to make this difference? It knows about which task is currently running and
which got runnable,
so it's not a big deal to know it's in interrupt context. From all the embedded
OSes I've used so far,
FreeRTOS is the only one to make this difference.
And in my view, passing the "fromISR" around up to portions of the code that
should be independent
of whether run from task or from ISR sometimes makes the code design worse.