While those patents are not enforceable in China (unless equivalents were also filed in China -- unsure if they would be worth much) they would be when imported to the US. This is one of the reasons the ITC exists, and it played a prominent role during the smartphone patent wars. So at least the US market would be protected from knock-offs.

The smartphone wars were fought among tech giants, not capital intensive hardware startups. The problem with patents is that you need to already be financially successful enough to file them, able to pay to protect them in court, and can float your company's operating costs long enough to see them enforced and rewarded, which may take years.

Yes and no -- filing patents is quite affordable (probably outdated info, but I recall average costs for drafting and filing was ~10K / patent, most of the costs being related to the drafting rather than filing.) Compared to all the other capital investments required for hardware startups, these costs are negligible.

But you're totally right that enforcing them is extremely expensive, slow and risky.

That said, Roomba isn't exactly a startup but wasn't a tech giant either, and did enforce their patents often.

And especially against imported infringing products, the ITC provides a much cheaper, faster mechanism to get protection via injunctions.

In theory, sure. In practice? Chinese companies ignore your patent, you waste money suing, it takes a long time.

If you win? Good luck collecting damages from China, and have fun suing the next brand that starts selling the same machine in different plastic

That's why the ITC is so relevant here: it is relatively quite speedy compared to regular patent trials, and have the power to issue injunctions against imports (which is partly why it was relied on a lot during the smartphone patent wars.) So you may not collect damages from Chinese companies, but you can completely block their infringing imports into the US and deny them US revenue.

Why isn't Amazon liable?

$ -> Lobbyists. Legal firepower.

Or, said another way: unwillingness to enforce.