What is the point of the website remembering what version the user is on?
So it can give you a more useful message. If you were on r130, and then you reload C3 for the first time in a while, it can say something like "you were on r130, and you got updated to r146". Now you know to check all the intervening release notes, if you want to stay up to date with all changes since then.
If there was a person that wanted to stay on r142. They would nav to editor.construct.net, get a message that the new version is release 142.3, then they would need to go to the address bar and type out editor.construct.net/r142 - then remember to go to that address every time they use C3.
The approach is based on the principle that most users will simply want to use the latest stable release, so it's geared towards that. The typical user will just type in/visit editor.construct.net, and if you do that, you're always using the latest stable release, which seems like a sensible default.
It's also our goal to make sure there's never a good reason to stick with old versions. Every update fixes a lot of bugs and we strive to maintain excellent backwards-compatibility. For the typical user, not being updated causes more problems than it solves - they typically end up running in to bugs we've already fixed (and sometimes reporting them, where all we can do is point out that we already fixed it, and they are causing problems for themselves by staying on old versions). So staying up-to-date is an important part of the UX, so people always get the best-quality software currently available. I think if we give everyone an opt-out of updates, rather than applying them automatically, it will result in a worse UX as a greater number of users run in to problems that have already been fixed. Modern software like Chrome adopts a similar approach of silent auto-updates for the same reason.
Of course software development isn't perfect, so sometimes people want to roll back - and you can do so simply by typing in the version number in the URL.
I think you've misunderstood how the URLs work, and presumably as a beta user you have more interest in rolling back versions occasionally. This is straightforward, since you can just type the version in the URL. If you visit editor.construct.net you're getting the stability-focused general consumer version. I am pretty confident that is the right approach for the majority of our users. If you opt in to beta releases I think you just need a little awareness of what the URLs mean, and then you can switch between versions at will.