I believe this happened in 2004, before the Stockwell execution. Nowadays they probably would have shot him, since it's been established that there aren't any significant repercussions for shooting innocent people.

This was intimidation, pure and simple. The Bill's best buddies, the Scientologists, are well aware of how much you can frighten someone simply by following someone around and making it very obvious to them that they're being followed, it's one of their favourite tactics for intimidating "fair game". (Comparisons with CCTV, which watches everyone, miss the point in this case - it's the personal touch that makes the difference.) If they really wanted a photo they could get one some other, much easier way.