Having worked as a CASA for many years, I can verify that this has been happening for years.
When the abuse is discovered, the child is either returned to the parent or moved to yet another foster home, usually one no better.
The case is most often ignored by CPS and the DA.
http://www.bostonglobe.com/metro/2017/12/07/scathing-audit-finds-department-children-and-families-failed-report-crimes-against-children/X0r2KnGGiNNXSGf4mh5q6L/story.html?s_campaign=breakingnews:newsletter