There's no reason to say an Android ROM is illegal. Or whatever semi-illegal might be. Maybe it's like semi-falling-off-a-cliff? Semi-hit-by-a-train? I don't mean to mock.
I always guessed Android ROMs were such a misery in the community aspect, because they're very easy and relatively useless. All you need is a computer, which is easy enough, then some open and free software, and you're ready to go and compile your own ROM. The source code is often available, and you can change a color here and there and maybe change some of the Linux kernel parameters so you can say it's for better battery life, or more free RAM if you're into that.
That's the easy part. Now the relatively useless part: phones don't last long. Someone who is keen on tweaking their phone will probably be looking for a new one every year, or every other year. At best they last about three years. The changes to the software is minimal at best, and every custom ROM is practically the same. There is no marketability in these ROMs. There's not millions of dollars of potential revenue to be gotten out of a server that has to run with 20 years of uptime, that a company can jump into and support, like you can see with other uses of Linux. Red Hat supports for like 10 years? Android devices get discarded in two. Therefore there is little professional attention to third party, open source distributions of Android.
Google develops it, companies alter it a little bit, then ship it. Hobbyists from all around the world take the open source aspects of this and maybe add some new stuff here and there. There's no need to expect professionalism from hobbyists.
(I find CyanogenMod, or LineageOS or something now, to be pleasant to use on a phone. It gives the option to run apps as root, and it comes with a terminal, and it tries to remain as open as it can.)