Because he can be claimed as a dependent, and the unemployment is unearned income.  But wait a few days and the unemployment should disappear, once IRS tells us how to do it.  

The Kiddie Tax can apply to non-dependents.

If the 21 year old is a full time student, the Kiddie Tax applies unless he pays for over 1/2 of his own support with EARNED income (or he files a joint return or both parents are dead).

So if the student is disqualified from being a dependent because he doesn't live with his parents, or because he provided over 1/2 of his own support but not with EARNED income, he would still be subject to the Kiddie Tax.

He is not a student and not claimed as a dependent on his parent's return.  

Is the IRS going to issue a statement about Unemployment?