Every department and authority has their own law enforcement branch. You've got county sheriffs, state police, highway patrol, campus police, transport police etc. on a local authority level.
Then on a federal level, as the paper points out, everyone from the IRS to the Department of Education have their own law enforcement officers.
In the rest of the world, police is much more centralised.
Here in New Zealand (admittedly a small country), the Police are the only agency with general powers of arrest, and the only department outside the military with firearms.
The police don't have fancy equipment, if they need an APC, or helicopters or whatever (the police do have one), they'll borrow it from the Defence Force. It's not like they need to pay for and maintain their own stock to use once in a blue moon.
If an agency like the IRD (our tax department) need to raid or arrest someone, they'll get the police to do it.
I don't fully understand why the USA isn't like this. I understand why there's federal and local law enforcement, but if the FDA needs to raid and arrest people, why don't they second some FBI agents when they're needed?