As far as I can tell you’re being dense. No one said that a ship couldn’t stop at the local gas giant and leave at will. Or anything else as long as they are not landing. You only have to stop at the starport if you’re going to land or transship cargo. Things like imperial banned technology or Quarantine would be two main reasons. Why do you think the imperium goes to the effort to have a starport as its embassy instead of just a building? Give me a way how the imperium polices things like importing slaves or preventing trade in WMDs both of which are illegal by imperial law. I guess in your Traveller universe people can ship in slaves no problem they just have to land at a spaceport kinda stupid of the imperium to go to all the cost of maintaining and supporting a starport if they are not going to use it. Not every thing is written out in black and white some things requires you to look at the whole picture and use common sense.As far as I can tell, you are just making this up. I have never seen anywhere any suggestion that the Imperium forces every ship to visit the starport. If you can find me one source that says ships have to visit the mainworld before going to the gas giant to refuel or some secondary world or space station, I'll be suitably shocked.
Now I don’t own the imperium source book so I can’t say it’s not stated there but it was mentioned in CT and just makes common sense