I presume the light comes from the factory with a short flex and either no plug or a foreign plug. I googled what I could read of the label and found what appears to be the same light with a moulded schuko plug
https://www.stagelamps.co.uk/store/led-par56-short-black. If quizzed about the plugs and sockets safety regulations the distributors will probably say the lights are "not intended for domestic use".
The customer then fits a plug or connector to suit their setup. In larger orgs this will be done by someone who hopefully has some level of electrical training, but there are lots of small bands, magicians, DJs and so-on out there who aren't going to have any specific electrical staff.
Why use the connectors backwards? maybe it was ignorance, maybe it was a deliberate ploy to stop people taking the leads. I have certainly got the impression that there are some in the entertainment industry with a somewhat cavalier attitude to electrical safety.