Blizzard has released a Hotfix for Season of Discovery which updates the Trap Launcher Rune to no longer apply to Frost Trap. This change was made because Hunters were gaining an unfair advantage in many PvP scenarios.
Trap Launcher Rune Guide — Season of Discovery
Without them, the safety and agency a 9-15 second ranged Frost Trap grants Hunters is simply too much. We're making it so that Hunters either need to put themselves at risk in order to place a Frost Trap or have someone engage on them to trigger it.
Please note: if you currently have the Rune equipped, you must unequip and reequip it, zone, or relog before you can use Frost Trap again..