Earlier quoted context omitted.
If private companies would start to imprison shoplifters, it would be illegal and they would be prosecuted. A private company is not government and government is not a private company. What is your point exactly?
https://www.wikihow.com/Legally-Detain-a-Shoplifter
Even if the state does it in NL, there is a difference between imprisonment and detainment. They are even physically different : they are different buildings.