All the policemen and prison guards in Sing seem to be rhinoceroses. The only exception I noticed are the pilots of police helicopters that are pigs. As far as I can tell other professional and social groups consist of animals of different species, so why the exception for the police force? Is it a reference to some American stereotype?
Probably budget reasons.
The film had a 75 million dollar budget, 15% of which went to acquiring the rights to songs used. For comparison, Coco cost 175 million; Finding Dory cost 200 million; Moana, Frozen and Zootopia cost 150 million; and Tangled cost $260 million.
If you're operating on a smaller budget, it makes sense to reuse character designs and their corresponding models and textures wherever it doesn't impact the story. The police force is secondary to the main plot, and are already somewhat of a uniform group, so they're a logical group of characters to save money on. My guess is that the helicopter pilots are pigs because it was easier to model and animate a pig piloting a helicopter than it would have been to make a rhino do it.