It depends on where they live and stuff, like if it's an English speaking country they might think the kid will be better off this way because they will learn English elsewhere. I would worry that during the early childhood years that they might miss important development though if they are only around their parents and only hearing Japanese.
Sure, they live in Japan so they don't NEED to be able to speak English. I look at it this way though. My cousin and his children visit the States maybe once a year to visit with family. How sad would it be if grandma couldn't get to know her own grandchildren because they can't communicate? I would assume the lady visits her family occasionally in the states.
If they live in Japan, they should be spending a significant amount of time and effort teaching their children English as well as Japanese. Not doing so is a huge disservice to them. ---