Because it isn't the governments that initiate the arbitrations. It's purely the investors who have the right to do that, and because they have the right to do that, if they consider a change in the law to have an economic impact on them, and if they consider in some way that change is protected against by the investment treaty, irrespective of the level of environmental protection in their home state—that's simply an irrelevant factor—then they can initiate the arbitration. European companies, if I may say, have initiated over half of the grand total of investment arbitrations today, so they're not strangers to the process and they're not shy about the process.
Evidence of meeting #11 for International Trade in the 41st Parliament, 2nd session. (The original version is on Parliament’s site, as are the minutes.) The winning word was canola.
A recording is available from Parliament.