"helps insiders and hurts everyone else" sounds bad, but if the insiders are being exploited by everyone else, it could be a good thing.

In general I'm sympathetic to your point. However I think Amazon is perhaps a special case, or at least occupies a different point on the than most companies.

Amazon is an Aggregator[1], which spins up its flywheel by providing a great experience for consumers, and commoditizing its suppliers. This naturally drives towards a monopoly.

Aggregator monopolies have an interesting property in that unlike historical non-Aggregator monopolies, they are great for consumers, rather than resulting in consumer harm through higher prices (like Comcast, say). US antitrust law is focused on preventing consumer harm, not just preserving competition for its own sake (as the EU laws are). So US antitrust is somewhat less well-equipped to deal with Aggregator monopolies.

If the corporation, consumers, and legal system are all pointing towards exploiting a group of workers, it's hard to see how anyone is going to vote to make things better. So unions could be more appropriate specifically in the case of Aggregators.

(Worth mentioning that you could perhaps just mandate higher minimum wages, and avoid the need for unions; I'd prefer to make the minimum viable change that prevents exploitation, and leave the market free to optimize everything else. But if not unions, who drives the political fight for minimum wages?)

[1]: https://stratechery.com/concept/aggregation-theory/