I think sunlight is more related to fire than grass, isn't it? I know plants can absorb light, but a beam of light. So, why is solar beam a grass type move?
Here are 2 reasons:
I would assume because it has to do with absorbing sunlight, and the only things that absorb sunlight would be plants. They use it for energy. So Game Freak must've thought, "What if they shot the energy back out in a concentrated beam of light?"
And voilà, you have Solarbeam.
Because the plants absorb the sunlight to survive. And the plant use this sunlight that is absorved to launch a beam.