The same way you can abuse paid labor, "can't they just quit?"
But there's no such thing with volunteering to try a new service. That's just something people do because they feel like it, are bored, enjoy it in their spare time?
To that, I will add that there is a large market for content outside mainstream media[^1]. I'm sure there are creative folk out there which are not visual artists as their main thing[^2], but can use cheap visual art coming from AI to generate some sort of income...
[^1]: See, for example, https://www.thinkwithgoogle.com/marketing-strategies/data-an...
It's an emerging technology and platform. There are economic advantages to having early access to learn what it can do.
Furthermore, this specific colunteer program appears to have an associated contest that pays creators who win. Thus you have a lot of volunteers, limited under NDA, being pushed to compete for vert limited renumeration for their efforts. I doubt most of the volunteers are bored people doing it as a hobby.
What do they lose when they weren't being paid in the first place?