Protesters are seriously against pipelines, as shown in Oregon. This is why it is a good idea to have guards at the North Dakota pipeline so that both workers and the pipeline itself are kept safe. If this means resorting to dogs, or even private guards, then so be it.
They were meant to make people back off while they slowly moved forward I assume. But technically on private property (if that's what it is) I think you can release guard dogs. The article here says they were not licensed which is the reason they are in trouble (at least the main reason).
Private guards should not be allowed against protestors. The protestors are making a stand against government issues. Private guards do not have to be regulated and can use excessive force. Especially in cases such as the Dakota pipeline protestors, where the protestors have been nothing but peaceful, the use of force is completely uncalled for.
No, I do not believe that private guards should be allowed to take a stance against the activists in North Dakota. Allowing law enforcement and other government agencies is a better way to handle these situations. Police are not biased and not on the siding with either party involved in the protest.