We're not very consistent with this though. We consider it perfectly acceptable to put people in harms way as part of the armed forces. Even though the exact same demographic effects are in play.
I doubt people on an ethics board for medical studies would find that perfectly acceptable.
Firefighters are a separate issue altogether, because the subject being discussed is a lot more ambiguous than firefighting (where the problem is clear, and the consequence for sending a firefighter is a positive result).
Many people go into harms way because of a necessary duty (firefighters, etc).
The restrictions on medical trials is because of the power imbalance. If I take a job as a firefighter or soldier that’s different from me living my life and being enrolled in some study putting me at risk for harm.
It's also entirely possible that some people would feel driven to participate in a medical study out of a sense of duty (e.g. I'm young and healthy and want to put myself at a bit of risk to contribute to our understanding of the virus so that my much more vulnerable (grand)parents have a better chance).