Given that they could just... keep track of the platform that installs/licenses are on, they'd know how many Windows machines are out there and how many Macs are out there. This doesn't strike me as a particularly reasonable argument - plenty of other companies manage this just fine. And, again - you're still talking two computers. I could have Macs with two different versions of macOS - would that not introduce the same sort of support headaches? And it's not as if this is a Mac-only software - they have thousands of users who are on Windows, presumably on everything from your HP All-in-Ones to Dell laptops to custom-built desktops. That's a lot of real-world experience running on that platform. Realistically, what kind of problems is someone running on one Windows machine going to run into that they haven't already run into on those thousands of other machines, which is isolated to them just because they happen to use a Windows PC AND a Mac?
Again, it's their software, they get to choose the licensing model - I'm just saying that it does seem arbitrary when most other groups can do this just fine. I pay $10/month for Photoshop and Lightroom, and I can put it on any computer I want. I purchase Civilization V on Steam, and I can run it on my PC or my Mac. Etc, etc.
EDIT: I should add, that the reason given was "we'd have no way to track license use between Windows PCs and Macs since we distribute through the App Store" - that's false. Not only DO they use license keys, they also distribute the software through a standard .dmg file if you so choose - I just bought Affinity Designer and my license key AND the .dmg installer is available on my account profile. ಠ_ಠ