> I read that it used to not be like this, that it used to be possible to renew the _visa_ itself from inside the US, but that got changed before my time. I can only imagine that the reason for that was that non-citizens inside the US are entitled to due process, but non-citizens outside the US are not. And denying a visa to somebody outside the US is therefore a lot easier than denying it to somebody inside the US, and essentially cannot be appealed

No, after 9/11 they passed a rule to always collect biometrics before issuing visas and validating them at border entry. The DoS facilities in the US did not have fingerprinting facilities but the consulates and embassies did, so they forced the change. Recently there was a pilot to allow it in the US itself.

This is just Trump trying to torture immigrants likely due to the psychopath Steven Miller.

In general the law applies equally to everyone associated with the US in any respect so you get due process (in theory) regardless. Specific laws may apply to different classes of people though (see 'enemy combatants').