Well--they are supposed to fight for the ball....
Head coach Ron Rivera had this to say:
"...One of the things (defensive backs) coach (Steve) Wilks is doing with his defensive backs is putting on the boxing mitts, and that way the guys aren’t grabbing. They’re using their arms and hands the right way and they’re developing some good habits, and we’re trying to break some old and bad habits.”
Rivera also mentioned that the league is planning on putting an emphasis on defensive players not making too much contact in the coming season. Practicing without being able to use their hands should go a long way towards helping that.
However, should one of the defensive backs get mad at someone during practice while wearing the gloves--comedy gold!