Be reasonable, all alternatives would have been considered. If they had let it be it'd keep causing a hazard to road users (potentially ending in the same outcome for the animal anyway), and shutting down a road and trying to contain a dog until a "council dog warden" (at 3am? Not likely) or other suitably-trained and equipped person can attend simply isn't practical.
It's not something the officers would have wanted to do, by any means, but they'd also be the first ones held responsible if it ran into traffic and caused an incident.
Yeah, if only they'd have had something to slow the traffic down and warn them of a potential hazard ahead like bright blue flashing lights and stuff....