Hi Marcus! Thank you for the well thought out and measured reply.
While spec work is certainly a culture of corporate America, its ethics are still in question, especially online. The large companies doing spec work have a certain financial backing and investment which makes them risk resistant, and independent game developers do not. Spec work is not viewed positively online, usually because most experienced game developers are ex-corporate. There has been significant backlash against people who have sought to do spec work via crowd sourcing on the internet in the past. Most notably, Shaq's request for prize-incentive animation.
It's reassuring to see that people are able to post their completed games. Even if we suppose that this moves the proposal outside of the definition of spec work, you are still having many users make a game to your specification (and political purposes) for free. Regardless of whether they're paid they're still filling the criteria you set out to invest in with your initial payment. (On a side, philosophical note, if people are making games for a monetary incentive, is the message still genuine?)
I can see that your decisions to do this jam are not malicious. I can imagine it's also very frustrating for you to be set so many questions when you just want to do something, right now, with the resources you have which will benefit the world at large.
I don't think you need to offer that publishing deal as part of the jam for people to make content. Most people are making games every day that fit the criteria you want to see. Many of these are on the front page of Itch.io or posted to Twitter under the relevant hashtags (usually a social movement paired with #gamedev or #indiedev). If you would like to amplify these messages, it might make sense to put some investment into people already doing work in the field.
If you do continue down the jam path, you should make it very clear what rights people do or do not have over the content they make, how the monetisation of the game will pay out and precisely what political messages you wish to make.