That's because half of the TPP comments on this kind of article are astroturfing by the Chinese, and the rest are suckers who don't realize they're along for the ride.
We're used to negotiations resulting in agreements that limit states between themselves; disarmament treaties, border negotiations, etc. Or things with a single well-telegraphed purpose (anti-drug treaties do what they say).
Treaties that bind the parties to enact domestic law that constrains what ordinary members of the public can do are another matter. It's a way to make law that cannot be reverted by a subsequent administration.
Do you realize that most of our trade treaties enact domestic law right? According to the current interpretation of article II, section 2, clause 2 the treaties are essentially a manifestation of domestic law (as was NAFTA, for example.) Where you are incorrect is in the assumption that it cannot be repealed. In fact, it is easier to get out of them than it is to approve them: congress can pass laws that repeal any aspects of the treaty (and no, a treaty cannot prevent a future congress from such action -- no act of congress is in any way binding on future congresses and only a change to the constitution would allow for such a restriction), the supreme court could declare any or all of a treaty as unconstitutional, or a future president can just declare that the US is no longer party to a treaty. The latter bit is still somewhat untested, but in cases where it has been used by sitting presidents it has not been successfully appealed.
However, Congress changing it subsequently puts the US in breach of the treaty, and potentially subject to whatever arbitration system was built into the treaty. The US generally has the power to get away with this without significant cost, but smaller parties to the treaty do not.
