Karajoma, the Shivans did not build a fence around GTVA space. The GTVA did it themselves.
"We want nodes!"
"But the Terrans and Vasudans are too strong... they will not allow us near their systems."
"Let's stop them spreading into other nodes then."
"How?"
"Send in a few of those civilian escape pods... what do you call them again? Oh, yeah, those Sathani that are so crap and small by modern Shivan standards that they were decommissioned years ago... dig a few up and send them into Capella to scare the **** out of the Terrans and Vasudans. That way maybe they will seal off their own nodes."
"Good idea, sir."
"Hey, look! This system is densely populated!"
"Let's just blow it up then, just out of spite."