Do kids HAVE to share?

I was at the park with my 2-almost 3 year old son and another little boy there (4) had a toy that my son wanted to play with. I told my son no, that it wasn't his toy and he didn't have to share if he didn't want to. The other little boys mom got so mad at her son for not giving up his toy, grabbed him by the arm, chewed him out, apologized and left. I was baffled. My son was a complete stranger to him and no, I don't think sharing is a MUST. do you make your kids share? Or give them the choice?