I never saw that until it started happening to Bonds' homeruns in the mid 2000s. Then somewhere along the lines it became common place to throw back opposing players homerun balls.
Knee pads.
Note: I mean for things like roller skating, cycling, etc. If you have a job where you are constantly on your knees, knee pads are perfectly acceptable.
It was one thing when people were throwing back Barry Bonds' home run balls because of the steroid scandal, but why do people throw the ball back on like any opposing player's home run? If you don't want it ask the people around you if they do.