I remember the watch party for the final episode of M*A*S*H. Twas in a Pizza Hut that was *packed*...FAR more so than even for Monday Night Football. And this was the heyday of MNF.
For *massive* spoilers...the last episode of season 3 of Downton Abbey. Or, the last Henry Blake episode of M*A*S*H, where he goes home. The endings of these, if announced in advance, not only color those episodes, but the entire series. The impacts on ratings, and therefore ad revenue, are huge. And in GoT, the potential that ANY character might bite the bullet, is a huge factor.
The privacy breaches are equally disturbing. Major celebrities have enormous problems maintaining any kind of personal space. In addition, of course, they are known to be wealthy, and thus would have big targets on themselves anyway.