I can’t say for certain, because I’m not familiar with how microwave radiation interacts with specific materials. I can say that the photoelectric effect (The only thing Einstein ever got a Nobel for) says that when any material absorbs a photon of the right frequency (yes, yes, I’m mixing the wave and particle properties of light, but it does that on it’s own) individual atoms can have their electrons excited, causing a voltage across a material, and a current flow due to the voltage, which means arcing, sparking and smoke. It’s actually quite possible to put (certain) metals in a microwave safely—depending on what you mean by safely. If you have the right set-up you can quite safely melt significant quantities of silver in microwave.
As far as the metal walls/roof, I believe Darwin is correct, but the entire assembly may act as a Faraday cage as well (and maybe a ground? though I doubt it), to prevent too much microwave radiation from leaking out. I’ve never specifically covered how faraday cages interact with electromagnetic radiation (as opposed to fields), but based on my understanding of E&M fields, and the nature of EM radiation: I’d say that a Faraday cage capability of blocking radiation is related to the wavelength and amplitude of the radiation. The floor, ceiling, and walls of a microwave are all metal, and have you ever looked closely at the front panel/window? The ones I’ve looked at have always had some kind of grate or plate with holes in it. My guess is that this grate is also metal and with the rest of the microwave forms a Faraday cage capable of blocking any detectable amount of radiation leakage.
All that said, a fair amount of science goes into designing microwaves, though you’ll probably never notice it. Their cavities are designed to set up a standing wave pattern of microwave radiation, causing your food to cook more evenly. (You can actually observe this pattern, with the right supplies. I’ve heard that a pan of mini-marshmallows will show a rectangular grid of melted/gooey marshmallows, a grid that will correspond with the antinodes of the standing wave pattern.