Building an easily-automated registration system that's "not for spammers" is like building a back door into an encryption scheme that's "not for foreign state-sponsored hackers".

Automation is a small part of the system and not the main concern. The main concern is making the experience of surfing the web less harrowing for surfers and to make it more smooth. There are many walled garden sites arbitrarily locking away information and that is counter intuitive to what the web is about, which is free access to information. In the true nature of the web, there would be mechanisms in place to unlock information, like a web browser is expected to unlock information. People's use of fake useragents that spoof a Googlebot to access Quora being a perfect example.

"Small parts" that are "not the main concern" are often very juicy targets for attackers.

Have you considered looking into Lastpass? There's a really great feature called "Form Fills" which allows you to save your personal information to a profile, and then filling out registration forms is as simple as right clicking on the first field -> clicking "Form Fill".

Yeah I tried Lastpass. I love that feature, but it is not a silver bullet and it often gets things wrong, populating the wrong fields and also still requiring some manual effort. I use Lastpass but I was spooked last year when their servers were (briefly) taken offline. Be careful of hosted services with all your mission critical passwords stored there. I now do local copies of LP when I get the chance.

Thanks for inspiring me to start locally backing up my LastPass encrypted databases.

Say I'm a spammer. What's stopping me using this?

Given that, why wouldn't website providers take active steps to block this?

I'd say browser auto compete solves 85% of this issue. The last 15% his the law of diminishing returns pretty quickly.