As a tangental comment to this discussion, I'm reading the Interdiction article in High Passage #4. There, it states that systems are typically interdicted through one of three agencies: The Imperial Navy; The Imperial Scouts; or by governmental decree.
The Navy interdicts worlds on the basis Imperial security concerns, quarantining planets perceived as threats to the realm (for a multitude of reasons). And, the Navy typically vigorously enforces these Red Zones. World interdicted by the Imperial Navy are typically completely off limits. Even visits to the system's gas giant can be achieved only on an emergency basis.
The Scouts typically interdict worlds on the basis of cultural reasons, either for the protection of the world or for the Imperium at large. Enforcement of Scout interdicted worlds is uneven and highly dependent on the reason the world was interdicted. The Scouts typically don't interfere with ships passing through the outersystem, away from the inderdicted world, and wilderness refueling at the system gas giant is typicaly permitted.
As for a governmental offical inderdicting a world, this is sometimes the result of a political maneuver. In any case, the governmantal agency/official that recommends interdiction must be of sector-wide influence or greater, but interdiction by this method is rare. It is suspected that these types of interdictions are sometimes influenced by the lobbyists of the MegaCorporations. Enforcement of worlds interdicted by these officials cannot be typefied. Almost anything goes. Sometimes the Imperial Navy is charged with guarding a system interdicted in such a manner, and sometimes interdiction is posted only by the routine signal of an interdiction beacon.
As a general rule, though, all interdicted systems should be avoided, even if entry into the system is for gas giant refueling purposes only.
If you're interested, there's more detailed info in the article. See High Passage #4, published by FASA.
