IMTU, you *usually* only see independent asteroid prospectors working the Lagrange Points of gas giants, where they've got a large collection of prospects relatively close together, It's only in places like Bowman Belt - e.g. those rare systems with unusually dense asteroid belts - that rockrats spread out into other system areas. And rockrats are usually quite able to protect themselves, at least against casual thieves. And casual thieves are the only threat they usually face, since the professional grades of lawbreakers know that any given rockrat just won't have enough in the way of loot to make the risk of taking that little it worth the trouble..

The mining ("processing" would be a more accurate term) of less dense regions is something for self-replicating, nearly-mindless robots to handle. There simply isn't a high-enough chance in such boring places for that "motherload hit" that attracts the direct efforts of sophonts. Only the collection/transhipment points (which are carefully kept secret by the megacorps who seed the aforesaid self-replicating robots) are worth hitting for thieves. This is discouraged by the fact that such points are generally booby-trapped with short-range space mines.

In systems along the Imperial border, those short-range space mines boast nuke warheads. These are licensed by the Imperium in return for the megacorps allowing their use in time of war as a space-denial system.

Again, IMTU.