Bad, bad!! You guys forgot your high school chemistry class already? C Minus!
Wire mesh, gauze or even a pad of steel wool will stop a flame from passing through. In the old class demo you held a piece of metal gauze over a bunsen burner, turned on the gas and lit it above the gauze, but it wouldn't pass down through it to the burner.
The flame trap does exactly what it says, it catches any flame that might possibly blow back from an explosion of gasses in the crankcase, preventing them from torching the top of the engine, carbs etc.
Explanation: The metal conducts away the heat of the flame and lowers it below the ignition point of the gas or gas/air mix. The same principle was used in the first safety lamp for mines, invented by Humphrey (later Sir Humphrey) Davey in 1815. Prior to this, miners just used candles, and explosions from methane gas were common. The Davey lamp surrounded the candle (and later, whale-oil lamp) with a cylinder of metal gauze. It was introduced in mines in 1816. Oddly, the famous engineer George Stephenson (he of the first sucessful steam locomotive) produced a similar design and introduced his in 1816 too, and for several years Davey and Stephenson accused the other of stealing their idea (this was in the days before patents). Obviously, Davey had better P.R., or we would be calling the device the Stephenson safety lamp.