It's not a gotcha. I'm not just saying "well, technically, $0 is a number, so that fits what you literally said". Limiting the price to $0 really is regulation of the price; setting the price to $0 is meant to alleviate some of the problems that could result from allowing a higher price, because it changes the incentives.
What do you think would happen if we duplicated the minimum wage structure in this arena? 'You can give away your kidney, or sell it for at least X dollars' where x is some amount deemed to reduce exploitation of the desperate? I like this idea.
Edit: saw comment below.
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