In the 1980s, it was common for vendors to offer source licenses to their customers, at times for an additional fee. DEC sold customers the OpenVMS sources (Compaq/HP continued the practice, and maybe VSI is keeping it up); IBM caused a lot of controversy in the early 1980s with the announcement that they were going to stop allowing customers source code access to MVS. Against that background, unsurprising that to learn that Sun sold source licenses to their customers for SunOS as well.