But is there a law forbidding strawberry futures? The wiki page mentions only onions and box office returns. How can other perishable futures "ban themselves" while onions need to be banned?
They're not banning themselves. There's no market for them. A market will arise based on the market size, product characteristics, etc. Without anyone willing to make markets and trade strawberries, there's no futures market for them. All that to say, there's no need for a law banning something if there's no willing market for it. There was an onions futures market and that's why the law is specific to onions.