Stranger Discipline?

[kitkatmc1]

My son is a very active 2 and half year old.  For the most part he plays well with other children, except for the occasional squabble over a toy.  Generally, if arguing over a toy, I let my son resolve it without trying to get involved, unless the situation turns into a major drama, in which case I will; and have no problem correcting him, if obviously he is in the wrong.  However this past weekend in 3 different scenarios of this same situation, the parent of the other child, (all strangers) approached my son, physically grabbed his wrist, and told him to stop and give the toy back.  Now in each situation, I was watching the whole dilemma play out and it was not my son's fault!  (And if it was, I would admit it).  I was thoroughly distraught with this and heartbroken as I watched my son walk away with his head down as if he had done something wrong.  Where do other's stand with this and how should I react to the other parent?