Jail for photographing police? (updated)

Jail for photographing police?, article.
"The relationship between photographers and police could worsen next month when new laws are introduced that allow for the arrest - and imprisonment - of anyone who takes pictures of officers 'likely to be useful to a person committing or preparing an act of terrorism'. [...]
A person found guilty of this offence could be liable to imprisonment for up to 10 years, and to a fine."

What strikes me is: how the hell can a policeman possibly know whether a photograph is "likely to be useful to a person committing or preparing an act of terrorism"?
The only thing he can judge for sure is whether he gets pissed off by being photographed, and you can be sure he'll act on that instead. And with new UK laws like this in place, he can't be penalized for it.

Update: here's a good article.