In order to accomplish this, your ZEN26 Switch uses two travelers to communicate with the regular on/off switches in multi-point control set ups (3-ways, 4-ways, and 5-ways - additional diagrams coming soon). As a result, there just wasn't enough space to fit in the bigger relays and hardware needed to control 15A.
Here are the maximum loads for the ZEN26 On/Off Switch:
It's never recommended to go over these maximums, otherwise your device will overload immediately or over time. Let us know if you're unsure if the lights you're about to connect are within the switch's specs, and if you have any other questions about your S2 Switch!