>>Why are we allowing the victims of shoplifting to hire armed guards

you have flipped the proposition from where it should be ethically. Everyone has the clear and natural right to defend themselves and their property from aggression including theft, We create governments in order to protect our rights, persons and property when government fails in its only job it reverts back to the individual to secure themselves and their property.

We do not "allow" people to defend themselves, it is inherent in their existence as humans we sometimes contract that protection out to government but never for a second believe we give up or surrender the right to do it ourselves.

>>sometimes abusing law abiding citizens?

I have no proof of this, and if they do they are no longer defending property and have become criminals themselves whom should be prosecuted as such

>>Isn't the state supposed to have a monopoly on violence?

The state has the self declared monopoly on the INITIATION of violence. Meaning the state has declared that is has the authority to attack you in any circumstance with or with out needing to justify their actions under the umbrella of self defense.

>>does that make these private police forces in public spaces ok

This is a problem with wording here, are they in "public" spaces or are they on private property that is open to the public? Do you think there is a difference between the 2? Based on the story linked to it appears these companies are paid to patrol shopping centers, malls etc. not "the streets" though they to talk about community outreach they do which seems to be different from their security patrols

Security services have been a business for all of time, do you also object to private armored car services banks use? or Security Guards at event venues?

I find nothing objectionable with their harsh enforcement of property crimes but I have a feeling our worldview on this topic will greatly differ as I am very very very much for very very harsh punishment for property theft. I cant stand thieves, and fully support self defense laws like the ones in Texas which allow people to defend their property with lethal force.

>>Or do we want a society where those with the resources can hire their own enforcers

I think a mix of both is the correct model. I certainly do not want my tax dollars to go to paying some cop to stand around the local walmart waiting for someone to steal something, Walmart should pay someone to protect their property. However private enforcement should be limited to stopping an act in progress and notifying law enforcement who then come in a reasonable time and actually investigate, and prosecute the crime.

>>opting out of normal police protection, leaving other more numerous victims to fend for themselves

This is a mischaracterization of what they are doing, the government is refusing to provide police protection, if the government refuses to provide it what should they do? just be victimized? Just take it? If the politicians say they will not punish people for theft, you believe people should just accept the theft? No that will never happen.

>>but there is a history of problems (like Pinkerton) related to such things.

yes there was, why? Because government did not have or refused to provide the resources to protect people and property, so like with anything the market found a solution. If you want to curb the possibility for abuses like this we need to support aggressive enforcement and protection of property. Failure to do so will ensure people and companies take the protection into their own hands which may not be good for anyone.

If you have a local DA that wants to stop prosecuting "misdemeanor" property crimes out of some altruistic or other reason I strongly encourage you to vote them out.