They probably are, and they probably should be arrested on general principles anyway, but Philly has a homicide problem and the police are concentrating on the easy safe stuff like this and meanwhile the murder rate in 2011 is outstripping the 2010 rate significantly.
Your logic is faulty. The fact they arrested these guys does not mean they were ignoring other crimes. They probably found out this crime while investigating another crime. Drug dealers use pit bulls to guard their dope. They can also fight them and make money.
DUH///
Its not either or...the connection to murder , violence against people is there with dog fighters.