Q: What is the reasoning behind allowing Warstomp to be useable in Tree of Life and Moonkin forms (assuming this is correct) but Bear and Cat forms have to shift out to use it?
A: You need to be in a humanoid form to do abilities like Warstomp. Tree of Life and Moonkin (and also things like Shadowform) are considered humanoid. They are bipedal for instance. This is a data construct more than a strict balance reason.
First off, tree of life is a treant, which is considered elemental, it can't stomp as it barely has the ability to lift up it's roots in the first place. Obviously you have to change it to humanoid status so that it's not OP against crowd control like sap and whatnot. That does not make it NOT an elemental.
Secondly, the bi-pedal restraint does not stay consistent with other mobs in the game that use the same skill. An example of this would be the infected kodo in BT that do a warstomp before dashing off toward to the kodo farmer. They have 4 legs and non-hoofy-stub looking things - whatever.
In all honesty the night elves get significantly more useful racials in both PvP and PvE. The NE "equivilent" would be shadowmeld which has many more PvE uses then warstomp, I don't know if/what it does in arena's and whatnot. We both get 2% nature resist, but the 2% evasion is a much better benefit then 5% BASE health (370 with racial, total health, diff story).
For any tauren class other then druid, warstomp has some decent uses. For tauren druid specifically - there's not a whole lot of bonus benefit.
"If you want to b1tch so much just roll night elf" - If night elves were horde I'd do it in a heart beat, if they made ANY OTHER RACE for Horde that could be druid, again, I'd do it in a heartbeat.