It does make sense in a twisted way to get rid of the young men to minimize competition.
They keep control in this community by making everyone afraid of the outside world, lying to them about what is out there waiting for them so when they turned all these boys out they were mostly terrified.
The young men, even turned out of the group, still believe Jeffs is a prophet of God and their salvation is still tied to him. Do you know how powerful that tool is?