I've been in this situation. Often we know within a few minutes whether the candidate is a good fit for our team, but for legal reasons we have to stretch out the interview so that the candidate can't make a claim against us. It would be better for both of us if we could cut them loose quickly, but we have to cover our butts because that's the kind of litigious society we live in.

And their is an initial phone interview, but it is done by HR or our boss, neither of whom are technically proficient.

Isn't that why a lot of companies do tech phone screens?

If it's because of anti-discrimination laws, can a technical phone screen be short? You could discriminate based on voice too so I'd think that'd still be an issue.

I think the advantage is that the phone screening is much shorter than an hour, for everyone.

And "you haven't passed this interview" is less suspect than "I ended this interview early"