That’s a recent change. According to the faq on the official forum, initially the idea was to charge every reinstallation. Then they realized it was crazy. Now it’s every first installations:
If a user reinstalls/redownloads a game / changes their hardware, will that count as multiple installs? A: We are not going to charge a fee for reinstalls. The spirit of this program is and has always been to charge for the first install and we have no desire to charge for the same person doing ongoing installs. (Updated, Sep 13)