Beyond the technological differences between Fedora and Red Hat Enterprise Linux, there are a variety of philosophical differences between them.
Yes, Fedora is the 'upstream' for Red Hat Enterprise Linux. However, every 6 months, there is a newer version of Fedora being published and after 2 releases, Fedora stops producing updates for releases. This means that if you are on Fedora 27 (Current is Fedora 28), when Fedora 29 is released, you'll no longer get updates produced for your Fedora 27 system. You will need to updgrade your system to Fedora 28 or Fedora 29 in order to keep maintaining the machine. So with Fedora, you're upgrading your system every 6 - 12 months. Realize that these are not necessarily trivial updates. In addition to an updated kernel (which granted, is a fairly slowly evolving software), you're getting updated programming libraries and languages, system managment software (e.g. systemd), and countless other packages that could behave differently than their earlier counterparts in previous versions.
While this newness is great for people who always want the latest and greatest stuff, it is not so great for people who need to support and maintain a platform for software that may not evolve as rapidly. Fedora also does not offer any type of commercial support, so if it breaks, you get to keep all the pieces. Fedora has 'spins', one of my favorites is the Fedora Games Spin, it has a whole bunch of Linux games already installed on it, so you can just load and go with it.
Enter, Red Hat Enterprise Linux. With major releases every several years and minor update releases every 12-18 months, the platform has much more longevity than Fedora. Add to that, a 10 year lifecycle for things like support and updates, and this is a much more approachable platform for people that need longevity and stability in their infrastructure. For example, if the company bought a piece of software that was certified to run on RHEL6, you can still, today, download and run RHEL6. Red Hat continues to publish updates for RHEL6. RHEL6 today operates in almost the exact same way as RHEL6 from 5 years ago. That Enterprise Database 11g rac installed on there is going to continue to operate from when it was installed until RHEL6 goes off the end of it's lifecycle. Also, when you call Enterprise Database for support, they won't laugh you off the phone because you're using one of their supported platforms.
As an administrator running business critical applications, this is the platform I want to use. I would hate my life if my enterprise decided I had to provide the same uptime and management and stability but had to do so on Fedora. (just the upgrade cycle on Fedora would make this almost impossible)
The thing in the middle is Community enterprise Operating Systems (CentOS). It used to be that RHEL was the upstream for CentOS, but increasingly it looks like it may be getting slotted inbetween Fedora and RHEL. The CentOS repos often have newer and different stuff than what's available for RHEL. It has more stability than Fedora, but has the same non-support (if it breaks, you keep the pieces).
It looks like the article linked above is out dated. Red Hat's developer program now offers a no cost developer subscription:
https://access.redhat.com/products/Red_Hat_Enterprise_Linux/Developer
-STM
