One of the big issues in a trade relationship involving multiple countries is what happens if a country unilaterally decides to ban a product, or raise import duties astronomically, or take a similar form of unilateral action that dramatically affects the viability of a foreign investor's project. This might go against something that the government itself contractually agreed with the investor. What does the investor do.
The investor can take the government of that country to court, but in many countries of the world there is no hope of winning, or it would take years in court. After all a government can simply change laws retrospectively (as India often does) and the courts can do little else but enforce them. It is precisely for this reason that the United States for many years has been insisting on independent forums for resolving Investor-State Disputes (ISDs). The US position has been that the legal system of every country outside the US cannot be trusted and therefore there must be an independent mechanism for resolving disputes. In current bilateral trade agreements with 5 of the 12 countries in TPP, the US already has ISD clauses. In recent years the US has an ISD mechanism in every trade agreement it has signed, bar the US-Australia one. In fact the biggest opponent of the ISD clause has been Australia, rather than the US. In a blatant double facedness, Australia has ISD clauses in trade agreements with developing countries, but refuses with developed countries.
The principle is not new either in the commercial arena or in governmental ones. Every commercial contract has arbitration clauses - parties submit to the jurisdiction of arbitrators rather than courts. This is both cost effective as well as time saving and is universally used in commercial contracts. There are well established global rules governing arbitration - the "capitals" of arbitration being London, New York and Singapore. If each commercial dispute came to the courts, the judicial system in every country in the world will come to a grinding halt - it is partly for this reason that courts themselves encourage arbitration.
It is therefore rich for US politicians, and especially Elizabeth Warren to argue against the ISD clause on grounds of loss of sovereignty. Firstly it is the US itself over successive Republican and Democrat administrations that has championed this principle. Secondly it the US which is usually the gainer in such matters - for example it prevents countries from outrightly nationalising companies and industries, as say for example, Argentina is wont to do.
Two cases are often used to illustrate how "greedy companies are milking countries" - the Veolia Egypt case and the Philip Morris Uruguay case.
Veolia , a French firm was executing a project to reduce greenhouse gases in Alexandria in Egypt. The firm executed a contract with the government whereby the government would compensate the company for cost increases because of governmental action. Egypt then raised the minimum wages in the country and Veolia then took the Alexandria authorities to arbitration for compensation for rising costs. The matter is in dispute and has not yet been decided. This is a contractual matter and the spin that Warren & Co are mouthing that this is a corporation suppressing minimum wages in a poor country is pure balderdash.
The Philip Morris case is more nuanced. Uruguay passed laws requiring that 80% of the pack contain graphic images and the risks of smoking. It raised taxes, banned advertising, and sponsorships. Philip Morris took this to arbitration on the grounds that this makes it virtually impossible to do business. The matter is yet to be decided. Uruguay is a signatory to an ISD arbitration and hence this came up before the arbitration panel rather than the courts in Uruguay. There is no evidence that just because it has gone to arbitration the ruling would be "unfair" or "unjust".
As a consequence of this case, in the TPP negotiations, the US has sought to prevent misuse of the arbitration clause by recognizing each country’s “inherent right” to regulate for health and safety. This will probably get incorporated into the final deal so that unilateral action by governments on grounds of health or safety cannot be legally challenged.
As far as the US is concerned, the TPP provisions are no different from the existing situation it already has in some 50 odd agreements. So why all this noise from Warren ? The noise is not because she has a better mechanism for dealing with an investor government dispute. It is in reality because she is against globalisation & trade. That is a different argument and battle.