Why shouldn't we? Does TPP prohibit that? No. Are you saying that countries with poor rule of law should not make any improvement to the Trent tho for the rule of law unless it does it all completely in one go? Are incremental improvements undesirable or actively bad? That's a pretty extreme absolutist position to take.
At the moment governments are pressured by other governments and multinationals to have stronger rule of law. The cost of capital is higher in more corrupt countries. The threat of capital flight is a significant disincentive to government crackdowns on democratic movements -- building stops, bond returns skyrocket, lines of credit disappear, arms sales stumble.
TPP is the promise that your foreign capital is safe, even though locals are screwed. Even better, companies sue governments for raising the minimum wage, or enforcing OH&S.
This isn't 'give a little, get a little', this is 'we're taking your rights'.
ISDR (investor state dispute resolution) is usually arranged as a private arbitration, with arbitrators who conspicuously do not meet conflict of interest or probity rules of even a hicksville kangaroo court. ISDR has the power to force judgements which are strongly not in the public interest, under the pretence of 'market interference'. The most obvious case is the tobacco lobby trying to prevent plain packaging legislation.
This idea that trade deals can guarantee commercial justice for large multinationals in authoritarian countries, but we shouldn't expect any such improvement for citizens, is quite abhorrent.