LIGO didn't announce because they want to be sure. But one of the most valuable aspects of LIGO is as a trigger for optical astronomy of transient, time-sensit

LIGO didn't announce because they want to be sure. But one of the most valuable aspects of LIGO is as a trigger for optical astronomy of transient, time-sensitive events. So, when they see something, they have to tell a wider astronomical community---the other observers who look in non-gravitational channels---though they might not be ready to make the information public. Each observatory comprises hundreds of people. Suddenly your message isn't contained to just the hundreds of people loyal to LIGO, but to thousands of people. Moreover, these observatories tend to be publicly owned and funded, and transparency and modern open-science practices often mean live updating of the status of these observatories. Finally, there are the scientists who have nothing to do with this physics, but fought hard for some observation time and had their scheduled observations interrupted for something high-priority. These people can infer (or are often told directly) why their allocation was preempted.