Techies like us get caught up in mechanism all the time in discussions like this.
But, though there are some explicit laws where that’s how it works, that’s not generally how the legal system works. If I have a private server, and I don’t give you permission to access it - or, even better, tell you not to, it doesn’t really matter how I secure it. If you access it, you’re in the wrong.
To give a physical analogy, it doesn’t matter how I’ve secured my house. Even if the door is open, you’re not allowed to just waltz in (or, to take it a bit further, come in and start using my stuff).
That is how I (a non-lawyer) understand it as well, but I wonder if it's so simple when you combine it with the GPLness of it all. Like, releasing something under the (A)GPL is a license to use and modify the code how you see fit, and that goes "virally" through the forks. This fork is just using their own GPL-licensed code, and it seems unreasonable (for some definition of "unreasonable") to limit forks in this way. I think it's plausible you can make an argument that if you make this kind of restriction in your GPL codebase, you're violating the GPL license of the original ("upstream") authors.
With no authentication it's a "gates down" scenario and it's assumed that if you put your server on the open internet you intend people to connect to it.
With authentication it's "gates up" and then "without authorization" from CFAA kicks in. I think it's unlikely that a user agent string creates a "gates up" situation, especially not if it's from code granted under a permissive license.
In general, I agree with you. However, to extend your analogy a bit further, so that it applies to _this_ situation: suppose you buy said house. When the former owner hands over the keys, you copy them. Then, one day, you enter the house using the copied key. The former owner can't really be all that upset, can they?
1. You bought the house. 2. They gave you a key, which implies that you have permission to use it. 3. Is the problem really the _copy_ of the key?