Correct me if I'm wrong, but they do not store previous versions for you to choose, only the current version.
But once you have the links, you can download the actual files directly from Apple, e.g., if you're ready to upgrade your Mac Plus to the latest and greatest version of System 6, download
https://download.info.apple.com/Apple_Support_Area/Apple_Sof...
https://download.info.apple.com/Apple_Support_Area/Apple_Sof...
https://download.info.apple.com/Apple_Support_Area/Apple_Sof...
https://download.info.apple.com/Apple_Support_Area/Apple_Sof...
[1] https://web.archive.org/web/20141025043714/http://www.info.a...
In this case, the existing license is only valid up to a certain point, after which you need to buy a subscription (because the perpetual license wasn't quite so perpetual).
We're talking 50 megabytes of storage here, per version. They can save themselves a lot of hosting money by letting lifetime subscribers download the latest version, but that'll cut into their profit margins of course. Even if they don't want to host the setup on their website, providing a 50MiB installer file on request has to be the bare minimum customer support I'd expect.
My experience is that some people do confuse FileZilla with Mozilla. That said, everyone I know just uses the free version, despite the spyware that's bundled with the installer.
Based on this post, I wouldn't buy their professional subscription.