Treaties don't actually bind governments all that much. Generally, there is pressure to sign them and not leave, but many governments agree to do things behind the scenes without formally signing a treaty.

So how is TPP different?

It's not. My point is what's binding is the internal and external pressure and TPP is almost irrelevant.

You'll notice that that was still New Zealand's decision.

How were they NZ's decision but TPP isn't?

Whether to sign up to the TPP is NZ's decision, but agreeing to it cedes power beyond just copyright, so withdrawing from it later is harder.

If NZ were to withdraw from the Berne Convention now, they would only suffer from the consequences in the realm of copyright. TPP appears to bundle several disparate things together, so there is serious leverage for others to prevent withdrawal if the treaty turns out to be significantly disadvantageous to NZ.