yeah, that particular flow is kind of intentional because that’s the commercial site. From the open source site (.org), there’s a link to download at the bottom of the homepage.
You can get to the open source site from the commercial site by selecting Why Concrete? > Open Source then following the link to the open source site.
Not disregarding any of the other comments about shortcomings of the cross-site navigation, though. Just pointing out that there is an intentional difference between the experience on .com vs .org similar to how competitors (ahem, WP) do it…