> US copyright law is extremely friendly to individual creators at the expense of corporations.

You must be joking! Yes, the "Work made for hire" requirements are convoluted, but most of the complexity applies only to independent contractors anyways. I have never heard of a US corporation be in danger because of an employee claiming copyright, yet people's personal projects and own companies constantly get screwed by former employers.

See, for example, the entire history of IP in comics, which Disney is now using to print money.

Yeah, about that, there was an artist named Ken Penders who worked on Archie's Sonic the Hedgehog comics. He sued Archie and Sega over the characters he created for the Sonic stories. Apparently work-for-hire due diligence was not done on Archie and Sega's part, because rather than squash him like a bug they settled out of court saying in effect: "go away, and take your stupid characters with you".

So apparently, companies do have to go through a few hoops to make their claim on their employees' IP airtight.

Please cite relevant statutes or case law. It's pretty easy to write a contract that only assigns copyright to the corporation for works that were created on company time. That is standard practice for pretty much every job, including non-tech ones.

When does company time start and end for a salaried employee? If I'm working on a side project and get paged to look at something for work, does work time start when I look at the alert? Am I on work time if I answer a work email on a work device with my personal laptop open doing side work at 3 am?

I mean, if it was done on non-Google hardware, off Google property, and using resources that you paid for / own, then I can't see any problems with that. They can still argue that it competes with their core business, but they would have to provide some justification if they wanted to fire you for cause or have the IP assigned to them. I guess they can just terminate you for no reason (I think?), but that's the risk you take.

The possible need to prove that an employee's work was done on company time (which is a vaguely-defined notion for US salary employees in the era of remote work) or with company hardware is a huge hassle for the employer, which can probably shed light on why they'd use blanket assume-ownership-of-copyright agreements, so long as employees are willing to sign such agreements.

Except Google is one of many corporations that exploit passion for the work their employees are doing to get hundreds of extra hours out of them off the clock.

You're right, it's very easy for them to say "only work at work" but they don't want to do that, so instead, anything created by employees is their IP.