She did? Like when? Certainly not in Crush when Tara is all "no more teleportation spells, honey" or Tough Love with "it frightens me how powerful you're getting.". Or Forever when Willow doesn't know what to do and Tara refuses to even contemplate the idea of resurrecting Joyce.
No, Tara tells her not to do everything with magic. Then Willow goes mental, wipes Tara's mind and it's Willow's idea to go one month without magic.
Those were all instances where Tara was not supportive; there are just as many shown where she is, and others where her support is implied.
Did not Tara participate when Willow was telepathically linking everyone to hunt vampires when Buffy was dead? If not openly support at least enable. Even going back to the episode Who Are You, it is Tara that encourages Willow to do bigger magic then they ever done before. It's Tara that thinks of it, and Tara that convinces Willow she can do it. Again, support and enable.
Was it not Tara who first 'violated' the group by making everyone "blind" to demons so they could not see her demon side in Family only to put the entire group at risk? It's not really that much different than what Willow did in Tabula Rasa?
Could Tara not have, while she was researching the leaf she found under her pillow found an "anti-magic" spell to encase the house? Or some mystic warning system. Dawn's little wish made a vengeance demon unable to teleport from the house, surely Tara and Giles could have found a way to ease Willows 'abuse' and 'reliance' on magic if only limited to the house. But...
When Willow conjured up all the party favors for the party thus saving time, did anyone show their disdain then? Did they refuse to profit from her magical interference? No. They ate, drank and enjoyed themselves, then later complained.
You cannot scold Pavlov's dog when you ring the dinner bell and it drools, you conditioned it as such. Willow was by no means a victim, but she was conditioned to be 'magic girl' and they did appreciate it, until her magic grew to a strength and abilities they did not feel comfortable with.
Walking out and in essence washing your hands of it, leaving it all to a living dead girl to sort, was the easy way out. She helped create the problem and did nothing to help solve it.