[rosser]

> Actually US citizens can legally perform arrests...

And in doing so, they are acting as self-deputized agents of the state. They're temporarily "arming" themselves with the state's power to use violence in furtherance of its (legitimate) interests in enforcing its laws. They could not legally do that without the state's sanction.

EDIT: Some countries don't even limit the power of "citizen's arrest" to their own citizens. The UK's relevant statute specifically says "any person", emphasis added, may act thusly.

[User23]

The rights that citizens protect with violence don’t generally require the state’s authority. I don’t need a law to give me the right to use force to defend myself and my property, that right is naturally mine. It is however very nice that the state’s laws generally respect those natural rights. Now the state can use citizens to enforce its laws irrespective of rights, that’s called conscription, as in for example a sheriff’s posse.