It is strange that I can't pick up junk with construction robots

I can change this in the next release, but it has some caveats. Right now the junk is using the "Corpse" prototype and that doesn't seem to allow deconstruction. I can change it what Rocks use which is "SimpleEntity", and that allows deconstruction by bots, but now it loses the functionality where it is automatically removed if you try to build over it. I don't see a way to get the best of both worlds. This change would make rail-building more tedious before you get construction bots.

Unless you are using mods that allow you to start on the forgeworld directly - you'll have access to bots. Most mods (all?) that restrict drops either allow you to drop construction bots (say Cerys) or give you access to them from the environment (Rubia). You can go Rubia way and add bots to some of the "corpses"...

I've made this change in 0.3.2, you can now deconstruct them with robots