That's an excellent point that didn't sink in with me until you explicitly stated it here.

For the benefit of the others, business class to/from Japan (or Korea) awards are 60k v 70k to other countries in Asia, so a 10k differential.

But, in the case of F, Japan awards are 80k v 110k for other countries, so a 30k differential.

Meanwhile standalone awards for Japan to other are 20k/25k/30k/40k (y/pe/j/f).

This means that it makes sense to pay 70k v 60k (even when the connecting flight is only available in Y), but 110k v 80k (+other) doesn't provide any savings unless the connecting flight is in F, which is a bit of a unicorn.

It's also worth mentioning that 20k/25k/30k usually isn't a great use of AA miles (e.g. TYO-China/HK/Taiwan is often less than $200 and SE Asia is less than $300).