They sent people with cameras after him simply because they didn't like him looking to dig up dirt and then they posted page after page displaying nothing but their own ignorance of alternative sexual activities.
To be fair, the videos in question weren't taken by the newspaper, they were filmed by one of the women participating in the orgy. She later sold them to the paper.
And the pivotal point of the case wasn'over the orgy itself, Mosley was angry that the incident was reported as being a "Nazi-themed" orgy, when it turns out it was just a regular old-fashioned whips-n-chains orgy, with no Nazi overtones.
Still, this guy's privacy was violated regardless. The paper got what it deserved. Fucking tabloids.