'Roasting green soybeans' isn't primarily about eating; eating is important but not the key point. The crux is that these soybeans are 'stolen'.
Those green soybeans are just the mature or nearly mature soybeans in the fields. Rural naughty children have all done this: while out playing behind their families' backs, they pull up the soybeans with the roots and then run to the roadside to roast and eat them.
This is actually in a primary school textbook—Mr. Zhou, a great writer, wrote an article about it, where naughty children 'stealed' soybeans and roasted them to eat—even if they are from their own fields, it still counts as stealing since it's done without the adults' knowledge.
However, the naughty children in the town are somewhat lacking. If they've never done farm work and can't even tell if the soybeans are ripe or not, then that just won't do.
