That is good advice that anyone engaged in the shooting sports
should heed!
Airsoft pellets aren't the only projectiles that have the potential
to ricochet back at the shooter--live ammo can do that too. A few
years ago, at the local indoor pistol range, a lady in the next
booth aimed her 22LR pistol a little high and hit the target
hanger. The projectile fragmented, and the small chunk of it came
back and hit me in the right shoulder. Fortunately, the fragment
had lost most of its energy and didn't penetrate skin. Then a
couple of weeks ago, we were at our club's outdoor range; I was
shooting, when my friend, who was standing beside me, said, "Ouch!"
and looked at his foot. Apparently, one of the 45ACP projectiles I
was shooting had bounced back and hit his foot. Fortunately again,
the projectile had lost most of its energy and didn't do any
harm.
That's why it's a good idea to always wear safety glasses when
shooting any kind of gun. 
