Why does NASCAR allow a cup regular to run the truck series??

It really irks me that Kyle Busch is so selfish he can't take his two rides and enjoy them. Why does he get to take a seat away from a newcomer who needs the time and experience behind the wheel to move forward? He has already reached the pinnacle of the sport racing in the Cup series. In my opinion it's the same as Clemens pitching in a high school game