This is wrong. It's a good rule of thumb because passive voice can be a common indicator of weak writing, but it has plenty of good uses in strong writing. The best case to apply it is when the object of a sentence was previously a subject or other focus of prior sentences, and so is more familiar to the reader than the subject of the new sentence. For example:

"The United States had maintained neutrality until 1941. However, it was attacked at Pearl Harbor, and then chose to enter the war."