I don't see how volunteering to enroll in a study (and being compensated for it) is any different to choosing to take a job as a soldier or firefighter.

Taking a job as a soldier is not at all comparable to taking a job as a fire fighter.

Both are expected to expose themselves to risk, both get paid. The many differences seem irrelevant to the topic at hand. What am I missing?

Soldiers tend to kill people, firefighters tend to save people, the difference - to me - would seem to be obvious.

That difference still seems to me not relevant to the topic at hand.

A printer creates documents and a paper shredder destroys them. Therefore, we can't compare the amount of power they draw from an AC outlet.

Soldiers and firefighters also certainly skew less wealthy/educated/powerful, so a priori I'd say the same power dynamics apply there (though it's of course possible that the effect is smaller if you dig into the numbers).

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).