That's being done on purpose, if they are editing the post after detection, they must be punished as 99.99% time, they post it knowingly.
Citation should be mandatory while posting contents from other authors. It can be a link, can be the name of the author or a book or whatever but a clear mention of author or direct link to the content.
Content I recalled this statement is from A Unfortunately, I can not find the source link for now"
At least, if you don’t exactly know the author of the content, it's the best to quote the text as a quote would simply refer to this has been shared by someone else. That's what the use of quote.
In general, I believe that people posting only the content from a source and having no further opinion of their own, should also be punished, maybe a warning.