I don't think the issue that people have is that they have to earn keys. The problem is that it's unclear how much work it takes to earn them. Lets go back to the sushi analogy.
You answer a quiz question at the booth, and get rewarded with a shiny, locked bento box with the promise of some sample sushi inside. You also get told that you'll get a key to open the bento box over the course of the meal for doing what you were already planning on doing: eating your dinner.
The first key arrives pretty quickly. You eagerly open it up to find some enticing sushi that you would like to try. BUT WAIT! You can't eat that piece of sushi just yet. You have to open up 3 more bento boxes, and trade in all 4 sushi samples if you want to enjoy one of them. Or you can choose to open up 2 more bento boxes, and trade in just 3 sushi samples for some other random sushi sample. Hopefully you don't get one that you're allergic to.
You're pretty good at the whole quiz game, so you start getting bento boxes lined up on your table. The keys are now coming slowly, but not nearly often enough to keep up with the bento boxes. You're not nearly as excited about the prospects of the sample sushi, especially now that you've learned that the sample of Banzai Kamikaze Samurai Roll is the most expensive sushi on the menu, so it'll actually take you 8 bento boxes worth of samples to trade in so you can enjoy that Banzai Kamikaze Samurai Roll.
You take a look at the 3 unopened bento boxes on the table that you're still waiting on keys for, and think about the other 3 you'll still need to get and how those will also need keys. That's when you notice that you can buy bento boxes and keys for a quarter each.
All of a sudden, it doesn't feel so free any more. It also doesn't feel like you're earning the keys, since they just seem to come out randomly. Maybe you should just buy some keys and bento boxes. After all, they're pretty cheap. In the end, it's like you're buying that Banzai Kamikaze Samurai Roll at a discount.
At least you got a Banzai Kamikaze Samurai Roll. The guy at the next table got an Asparagus Roll from his bento box. He said he hates asparagus. He didn't even bother answering any more quiz questions after that.