A power supply has a pair of connections for mains input (it may also have a connection for mains earth). While one of these connections is nominally neutral portable appliance standards generally assume that both of them are potentially dangerous. Reasons for this include the potential unreliability of flexible wiring and frequently plugged connectors and the fact that some countries use unpolaised sockets.
"refrenced to mains" means that there is a low impedance connection between the input connections and the output such that hazardous voltages/currents* on the input may be passed to the output. So the output terminals are a potential shock hazard. Such power supplies are cheaper to build as no transformer is needed.
It is possible to design equipment with transformerless power supplies safely but it means treating anything connected to the output of the power supply as-if it was connected to live mains.
I doubt that the cables and connectors on those lights would pass muster as mains connectors at least in Europe. The lights themselves it's hard to tell as it depends very much on how thick the plastic is.
User expectation is also an issue. Safety standards often talk about "reasonablly forseeable misuse". If something looks like a typical wall wart then people are going to assume it is a typical wall wart. Even if the equipment supplied with the non-isolated supply meets insulation and touchproofing standards for mains it would be all too easy for the wall wart to get repurposed to supply something else.
* to shock someone requires both sufficient open circuit voltage and sufficient short circuit current.