Quite an exploit/bug, you can leave other walls unclaimed to have them left by the raiders.
File this under "don't fix" please. :D
It's like hanging an "Armory" sign on it. Brilliant.
Basicly this is nothing new and allways happened at previous releases too.
Hmm, more of an AI derp/exploit than a bug. Definitely I'll take it as a note on the exploit list. Thanks for reporting!