I let my children roam free around the playground and parks . I like for my children to discover things on their own without me being in their face or getting in their way. But with that I keep my eyes on them and if they are about to go somewhere or do something danergous I either tell them to move away and becareful or remove them from the situation.
I honestly think that parent was rude. There is no reason for her behaviour. She saw your daughter coming she couldve slowed the swing down. That's what I normally do when I am pushing my daughter on the swing I see a child near by and heading our way I slow it down to be safe. After all I dont know what the childs intentions are or where the child is heading.