Well, well. Surprise surprise.
Diana was a most unpopular figure with the (US and European) arms manufacturing industry:she was the world's most 'visible" woman, and she was heavily involved with the global movement to ban anti-personnel and other types of landmines: landmines are of little military use; the huge majority of people who are maimed and killed by these weapons are civilians. Her actions in spreading awareness of the effects of these types of weapons was extremely bad PR for a number of corporations which are not only household names, but also profit handsomely from the purveying of death and destruction.
Diana was just one of many people who were trying do something decent and moral in an uncaring world ,on behalf of civilized values and humanity, and end up being targeted by those who don't give a flying fvck about anything other than power and profit margins.
It's hardly surprising that she was "spied upon". Business as usual, whether if be under Clinton, Bush, or whoever.