From a Brewmaster point of view:
Brewmasters are using one of two resources. Chi or Energy.
If we have no chi, and no energy... we're stuck spamming Tiger Palm, since it costs absolutely nothing for Brewmasters.
WW have this problem a lot less often, what with FoF giving them time to pool energy back up, so they're rarely stuck with no button to push.
Mistweavers would be REALLY weird if TP had a mana cost instead- 5 Tiger Palms to make your next Surging Mists free... but Tiger Palm costs mana. Mistweavers would eventually be faced with having to solve this equation:
Is ((tiger palm mana cost * 5) + (tiger palm eminence healing * 5) + (surging mist healing * 1)) > ((surging mist mana cost * 1) + (surging mist healing * 1))?
I realize that's not a direct equation as we're trying to add two non-comparable things (mana cost and heals), but you get the idea. Mistweavers would need to redo the math every time they got new gear to see if it's more mana efficient to use TP instead of just straight up healing with SM- or maybe even see if it's more worthwhile to only stack a few TPs before using SM, or... It'd add unnecessary complication to Mistweaving.
Brewmasters need a free spell for their empty GCDs.
Windwalkers rarely have an empty GCD so don't need the free spell of Brewmasters, and would have very little to gain from having their only choice for chi consumption being BoK 90% of the time.
Mistweavers... don't even think about it.