Simply because he wanted to take advantage of The Chipmunks. As you might recall, Ian is initially nice to the boys; buying them new toys and letting them have all the fun they wish. Ian began to work them hard and hard to increase profits, regardless of the price. When the boys decided they had enough, Ian's only choice to keep his profits (as the real world unfortunately works most of the time when it comes to money) was to try and imprison them. He tried a similar method on The Chipettes in the second film only before reforming in the third film. Ian lied and abused both of the groups for selfish gain. Ian's behavior shows exactly how many people can act once their prosperity is threatened.