1 No one knows the future but God
According to the Quran , Muslims' holy book, no one can know the future or predict except God. Muslims also believe that God can reveal a part of the future to some of the prophets he sends.
2 Islam is against Future telling
If people used astrology to predict the future or to claim that they know what will happen next then they will be considered liars by Muslims and they will be also be regarded as sinners.
3 It it's OK if it doesn't refer to the future
If Astrology is used as a science to tell how people's personalities are like or how certain factors in the universe affect their behavior then this is considered OK as there is nothing in Islam that goes against science.
4 Islam encourages science and learning
Many of the verses in the Quran encourage people to learn and to study various sciences in order to understand the origin of the universe and how the things in the world work. Islam is a religion that encourages science and learning. (See What are the benefits of studying a foreign language?)
5 Predicting the future is OK
Predicting something in the future by assuming that it is likely to happen because of scientific reasons is OK. What is condemned in Islam is claiming that a person knows the future. For example it's OK to try to predict when an earthquake might happen as long as it's just an assumption.
6 Muslims have a direct order to study the universe
In the Quran Muslims were given a direct order to study the universe and it's origin. The Quran says 'Say, Roam the earth, and observe how He originated the creation. Then God will bring about the next existence. God has power over all things.' Sura 29 , Verse 20.
7 Astrology was taken up by Islamic scholars
According to Wikipedia Astrology was taken up by Islamic scholars. The part of Astrology that was probably studied was the one not related to future telling. (See 10 Most Popular Zodiac Signs In Astrology)