There's a lot of people up in arms about this–either because they are mad because Apple is forcing employees to merge their accounts, or that they think this is completely false because this is not strictly required. As always, the real truth is somewhere in the middle.
Apple does not mandate that you use your personal iCloud on your work device. You can absolutely make a fresh account (from a new Gmail or something) and use that to sign in, and then throw it away once you have left. Some employees do this: it's possible, but fairly difficult.
However, it is also true that many (dare I say most?) employees do sign into their work devices with personal Apple IDs. Why? Because there are several fairly strong pressures for them to do so. New hire training has natural variation, but often the person will tell you to "just sign in" to when the iCloud screen shows up on your new computer, without explaining (sometimes not even knowing) that this doesn't have to be an account you actually use, and many people do so thinking it is required to go forwards in the setup process. And Apple rewards you for linking your account: you'll get iCloud storage and access to all of Apple's apps for free.
But that's not all: there's a very strong culture at Apple of "dogfooding": using the beta software for yourself to try it out and file bugs against it. Doing this effectively all but requires having an account you actually use, and for many this is the main reason why they only have one work phone rather than two phones. Plus, on Apple devices if you're not signed into an account that you share elsewhere, a lot of things will not work: your Apple Watch will not sync or unlock the device, you can't listen to your Apple Music, the iMessages your coworker sends you to pick a spot of lunch are not going to be seen.
Obviously, it is always easy to tell an employee that they should trade privacy for convenience. But my point is that the culture explicitly pushes you in this direction, and Apple themselves are pretty bad about employee privacy. They've developed a full BYOD MDM solution that keeps your work data separate from your personal data, so companies can just access or delete that, but they don't actually use it internally. And you think you're being cool with your two iPhones that you bring to work? Nope, any device you bring onto the premises is subject to search, even if you have never MDMed it and only use it for personal work. So they, too, are way behind on where they should be with regards to maximizing privacy. It's regrettable that this employee found this out the hard way–but she certainly will not be the last, and I think it's entirely valid to call out Apple for not moving forwards as other companies have.