I'll tell you the answer. . . they jumped right in there with what they perceived to be an equally attractive other toy to offer, so both kids would have something to play with.  And so both kids dropped the previously desired toy in favor of fighting over the new one.

I always tried not to do that.  I think mostly I was successful.  Unless he was threatening to draw blood (or having his drawn), I let my son and his warring playmate figure some things out on their own. 

Like, if you tug hard enough on something and force the other person to let go, there's a real good chance you'll land on your ass.

Like, playing together is supposed to be a mutually fun experience, not a battle of wills.

Like, if you exhibit this type of selfish behavior too frequently, you won't have anyone to play with.

But the helicopter parent wants to spare his/her baby those lessons in life, and so cheerfully martyrs him/herself for the rest of the offspring's natural life.

Because they usually don't stop when the kid reaches adulthood.  They are always there with the checkbook, always making excuses for the kid's bad behavior, and always, but always micromanaging the details of their adored child's life.  No matter that the child is now an adult.