I'm using a router distribution -- no names please -- where the firmware build process is so intricate that their documented way to build LTS .iso images is via a docker based system. I've had very little luck (until recently) in getting that to actually work.
They provide nightly .iso images, but charge an arm-and-a-leg for the LTS images.
I've tried to use their nightlies, but they were subject to considerable churn in the last few months. That resulted in broken firewalls/routers.
I will NOT trust my firewalls to that, sorry.
In their defense, they _do_ have a way for "community members" (i.e. folks unwilling to pay $1000s/year for a stable build) to gain access to the LTS images. However the bar they have set to gain access is so high that my filing a bug they graded as "high priority" and then tracking down and verifying the workaround for a release candidate didn't qualify me to access the images.
I agree there is a gray area, and that the core developers deserve to be paid. I am actually willing to pay them for their software, but not $1000s/year.
In my honest opinion, they have crossed a line.
(Edited: changed "fix" to "workaround" above.)
What you are referring to is often labelled shared source. For instance, Microsoft offers the Windows source code to some customers for reference purposes, but that source is not allowed to be built or distributed.
It's fine to build a business model around your open source project. It's fine to charge for builds as your business model.
What is not fine is purposefully obfuscating the process needed to build that open source software to encourage users to just pay for the builds. That's shady and counter to the spirit of open source.