Because... they don't want other survivors to wander into irradiated areas and die horribly? Y'know, something like that? I guess they could've left a voice mail.
Seriously, though. Yes, low on resources, low on manpower, you still had a handful of people desperate to keep any more casualties from happening where there had already been a lot of death. If all you can do is leave a sign that says 'don't go there', you at least did /something/.
Maybe the Brotherhood set them up ? Could be that they've found a stash of them somewhere and decided to mark some of the most dangerous areas with them.
To those craters in F2:
Ive checked wikipedia and it seems that there is indeed something important there. The area where the craters are is Sacramento Valley which is the northern part of the bigger Central Valley.
It may look like a flat area full of nothing but it isnt.
In fact, Central Valley is one of the world's most productive agricultural regions ! It makes up 8 percent of the USA's agricultural output. So creating a massive amount of Fallout there would contaminate untold masses of farmland making it useless.
It is also a major hub for the Union Pacific Railroad system and a few important highways like the Interstate 5 go through it. Destroying the area would propably mean crippling the backbone of the west coast's transportation infrastructure. So its indeed a strategic target.