Sometimes a common error, as people just have a rather ordinary interpretation on the meaning of the word "theory" and sometimes it's an intentional attempt of discrediting.

Words can mean different things in different contexts. A scientific theory is not the same as the general or ordinary every-day meaning of "theory".

Classic example and mistake by followers of creationist religions: "evolution is just a theory".

Well, what if I told you, that, for example, our modern electronic means of communication are part of the wide field of "information theory"?