The study of psychology itself can be scientific. The problem comes when it's not conducted properly, when the psychologists don't follow the scientific rules. However, that is insufficient to say that psychology is 'not a science'. It is a huge generalisation to say, from a fairly small research, that because some people don't apply the amount of 'scientific rigour' that the entire study should be invalidated.
Whilst the claim may still be true, it should be understood that psychology [and pretty much all of life sciences and social sciences] do not have as much a reliance on axioms and logic like physics and mathematics. They, however, still follow the same scientific process, from forming a hypothesis, collecting data, reviewing data, reform a theory, start again. It is falsifiable, a theory can be chucked out of a window, better models are produced, just like any of the other 'hard sciences'.
But sometimes some psychologists [even the famous ones] skip the collecting and reviewing data part, and just likes to write a lot of theory.