A lot of people have had a knee-jerk reaction to the jam. $12,000 is a lot of money for an indie dev! Of course it is. It's clear Marcus has a good intention for an investment to these causes.
Here is why it's spec work and not a jam.
Say you needed a wooden bridge building. $12,000 is a lot of money, but you want to make sure you get the best bridge for that money. So you ask a thousand people to build you a wooden bridge first so you can test them all out. The bridge is a very specific specification that can only be used in one place. Then you pick one of those people and make them rich.
However, there are now 999 people who are unpaid with a bridge they can do nothing with - in this case, a game with very specific characters designed to a very specific story.
In very real terms, you are asking people to dedicate a large amount of time to your project so that you can pick the best one. While probably not intentional, this is ethically poor, especially when you consider that you're doing it against a backdrop of issues people care very deeply for.
How to fix this: remove the tight specifications, remove the marketability of the final product, remove the microtransactions which pay off the $12,000. Currently this is not a game jam with prize money. It's spec work with an initial investment.
Video explaining why spec work is unethical: