The same reason people and organizations prepare for the worst. Because it will happen, as it did in this situation. Some people don't think rationally at times.
Yeah, but still... I could technically extend that reasoning to many other cases, and thus remove any possible course of action by the police for stopping criminals.
While the theft was minuscule, sealing of an escape route by opening/closing a bridge doesn't seem like a terribly bad idea to me. It of course depends a bit on how much time the thieves had to react, but from the sound of it, they actively chose to try to jump the bridge.
If the raised bridge meant that they were on a dead end road, they police could surely have slowed down to give them more time to think, if that would have been applicable.
A clearly marked dead end isn't something that would normally jump out at a driver, is it? I assume that the bridge was illuminated even though it was dark, so it can't have been a surprise that the bridge was raised.
The theft was in no way big enough for the perpetrators to take such a risk, and the punishment would probably have been pretty mild as well. It could have justifiably been assumed by the police that the thieves wouldn't risk their lives on a raised bridge.
I really don't know. Unless this particular policeman has a history of haphazardly putting people in unnecessary risk, I would probably put this down to simply bad luck.