< I have yet to see a convincing argument, here on HN or elsewhere, about how these companies are taking advantage of workers.

Some issues referenced in the link the FTC intends to address:

Misrepresentations about the nature of gig work:
While gig companies promote independence to potential workers,
in practice these firms may tightly prescribe and control their
workers’ tasks in ways that run counter to the promise of
independence and an alternative to traditional jobs.

Holding companies accountable for claims and conduct about costs and
benefits:
Gig companies must not be deceptive in their claims to prospective
gig workers about potential earnings, and they must be
transparent and truthful about costs borne by workers.

You may object that workers can just quit if they find the work unsatisfactory, but people often commit large investments of time or money to get these jobs. As a personal example, I once worked as a google search query evaluator. The job was advertised as paying $14.50 per hour, and you could work at any time. It required studying a ~100-150 page manual of search query evaluation minutiae, and passing a test demonstrating mastery of the material. Once I started, I found out the advertisement was a misrepresentation. You would periodically be given some allotment of work, it could be several hours worth, it could be two minutes worth. You might go weeks without being assigned work, and you had to login to check if any had been added to your queue. To be paid you would submit a timesheet weekly, where you would note each type of task completed, alongside the count, and the time spent. There were target rates and minimum accuracy levels for each type of task. Failure to meet accuracy requirements resulted in termination. If you submitted a timesheet where the calculated rate was lower than the target rate for any category of task, the timesheet would be flagged, and you would be enjoined to ‘correct’ it. I’m sure most people simply lied and decreased the time billed in these situations, accepting tacitly a wage far below that represented to them. The thing is the target rate for most tasks was impossible. It wasn’t based on people’s ability to complete tasks, but by the degree to which they could be squeezed.

Concentrated markets: Markets populated by gig companies are
often concentrated, resulting in reduced choice for workers,
customers, and businesses. These companies may be more likely
to exert their market power in anticompetitive ways that
harm workers’ wages, job quality, and other aspects of gig work.

Combating unlawful practices and constraints imposed on workers:
Gig companies using artificial intelligence or other advanced
technologies to govern workers’ pay, performance, and work
assignments are still required to keep promises they make to
workers. Companies must also ensure that any restrictive contract
terms, including those limiting workers from seeking other jobs,
do not violate the FTC Act or other laws.

Policing unfair methods of competition that harm gig workers:
The FTC will investigate evidence of agreements between gig
companies to illegally fix wages, benefits, or fees for gig
workers that should be open to competition. The FTC will also
investigate exclusionary or predatory conduct that could cause
harm to customers or reduced compensation or poorer working
conditions for gig workers.