"UL" = equals minimum safety standards to be met (by them, the laboratory). Just like NEC (by NFPA). We here give you our suggestions from "field" experience. This is why I "liked" nealtw's reply.
Yes, back stabbing is approved. However, by field experienced persons such as myself (no, I am by far not an expert!!!!!) find many times that back stabbing receptacles or switches serve to be an issue in the future. The pressure applied to any electrical connection is the secret to keeping that connection "safe!!". "Loose wires start fires" is what my instructor always preached. It is true too. Many times loose wires will arc and cause heat, failure and or fire in time. When you put a wire in the back-stab location/slot and then push the light switch, receptacle etc back into the box wire tend to move, bend etc. The pressure applied to the piece of metal that holds the wire/conductor in place in a back stab is not sufficient over time to hold a tight connection. It may not fail at that moment but over time may start to slip out of the connection and cause a failure such as no connection or worse, an arcing. It is highly recommended that all conductors be placed under a screw (only one wire per screw) and tightened sufficiently with a proper sized screwdriver.