In my Choctaw catechism it says that believing in dreams is the same as believing in fortune tellers and talismans. Why is believing in dreams a sin?

Choctaw catechism?

The Choctaw are American Indians mainly in, I believe, the southeastern United States.  I presume that they may have been relocated by U.S. authorities on occasion.  Prior to being introduced to Catholicism they would have had a pagan religion of some sort.  Perhaps dreams are an essential part of that pagan religion so, in order to help former practitioners of that form of idolatry the writers of the Catechism were very strict to condemn that means of communing with the Evil One.

In the First Letter of St. Paul to the Corinthians, St. Paul condemns the eating of meat sacrificed to idols.  He specifically says that it has no meaning for the idols are false gods and have no power, yet he says that the Christians must refuse to eat such meats in order to preserve the faith of the weak and to show that the Christian God is the True God.

It seems likely that the writers of the Choctaw catechism may have been teaching the same thing in this false religion in regards to dreams that St. Paul taught in regards to meat.

'The dreams which appear to the soul through God's love are unerring criteria of its health. Such dreams do not change from one shape to another; they do not shock our inward sense, resound with laughter or suddenly become threatening. But with great gentleness they approach the soul and fill it with spiritual gladness. As a result, even after the body has woken up, the soul longs to recapture the joy given to it by the dream. Demonic fantasies, however, are just the opposite: they do not keep the same shape or maintain a constant form for long. For what the demons do not possess as their chosen mode of life, but merely assume because of their inherent deceitfulness, is not able to satisfy them for very long. They shout and menace, often transforming themselves into soldiers and sometimes deafening the soul with their cries. But the intellect, when pure, recognizes them for what they are and awakes the body from its dreams. Sometimes it even feels joy at having been able to see through their tricks; indeed it often challenges them during the dream itself and thus provokes them to great anger. There are, however, times when even good dreams do not bring joy to the soul, but produce in it a sweet sadness and tears unaccompanied by grief. But this happens only to those who are far advanced in humility.

We have now explained the distinction between good and bad dreams, as we ourselves heard it from those with experience. In our quest for purity, however, the safest rule is never to trust to anything that appears to us in our dreams. For dreams are generally nothing more than images reflecting our wandering thoughts, or else they are the mockery of demons. And if ever God in His goodness were to send us some vision and we were to refuse it, our beloved Lord Jesus would not be angry with us, for He would know we were acting in this way because of the tricks of the demons. Although the distinction between types of dreams established above is precise, it sometimes happens that when the soul has been sullied by an unperceived beguilement - something from which no one, it seems to me, is exempt - it loses its sense of accurate discrimination and mistakes bad dreams for good.'

St. Diadochos of Photiki