There already is a law in Thailand requiring the taxi driver turn on the meter. I just checked and found many sites online confirming this, and I think it's on the paper you get at the airport when you take a cab from the stand, which is another good reason a first timer should use the taxi stand. Drivers refusing to use the meter should be reported. Did a driver tell you he didn't have to turn on the meter?

And you seem to not understand how GrabTaxi works. The app sends a cab to where you are, as opposed to your flagging a cab down. Regular cabs participate. It's not like Uber where it is a separate system. Flagging down a taxi that participates in Grab is no different than flagging down any other cab. They are the same cabs. You might be flagging down a different cab company's car, but that has nothing to do with the cab participating in Grab. You could not flag down an Uber. Uber is a company with its own system of cars and drivers that are not usually cabs. (They can be in some countries.) With Grab, you are using an app to call a cab, but it will be a licensed cab driven by a cab driver as opposed to a car or driver affiliated with Grab.