One word: MONEY! That's all there is to it. It's sad, but it's just the way things are. The word "invade" works fine in your example because it's truly an invasion of privacy.

That is the cost of celebrity, isn't it? They're celebrities because they get a lot of publicity, wanted and unwanted.

pure business...boost circulation...appeal to ads companies...all come down to money,by the way, ‘invade' is not appropriate here, try'infringe' or 'vialate'

Because stupid people pay for magazines that print the photos.