Foster parents are created any time a node from the referencing scene is parented to a referenced node. That is, if you reference geometry, and then parent something under it, a foster parent node is created when you unload your reference.
This is how referencing ensures that parenting is properly reapplied when you reload a reference. Since files are saved with references unloaded, both export and save automatically generates fosterParent nodes.
To export the file without any foster parent nodes, you can export with Preserve References turned off. However, this bakes all referenced nodes into the exported scene file.
Foster parent nodes only exist for the duration the reference is unloaded; they do not persist after the reference has been reloaded.
