AZ has no law allowing a motorcycle to turn left no matter how long the light stays red, though at many intersections they are replacing red arrows with flashing red or yellow arrows, allowing anyone to make a left turn on red as long as they yield to all other traffic.
My city's traffic engineer says a magnetic field would indeed trip an inductive grid, bur it would have to be an enormous one, akin to one of those electromagnets that junkyards use to pick up cars with. The grids are designed to detect magnetic surface area. And it has to be fairly close to the ground. A large car would be the best thing. Even huge lifted trucks sometimes have issues with these things, because even though they contain enough metal over a larger enough area, that metal may be too far away (too high) from the grid to be detected.
For that same reason, in order for them to work, they cannot be buried deep in the asphalt, where they would be protected, but must be near the surface, where they quickly become damaged by traffic. The more segments of the grid that are lost, the more difficult they are to trip. And because repairing/replacing them requires ripping up the street, which means closing it, that rarely ever gets done. Fortunately these things are slowly being phased out.