I had always considered the SPA's control to be of the "order" rather than "law" type. If you carry your gauss rifle across the XT line onto a LL8 planet, you will be arrested. Said gauss rifle does not offend imperial law, however, and will not get you arrested inside the starport; the SPA can tell you when, if, and even how you may carry it openly throughout the starport, however. The emphasis is one letting everyone safely through the starport, not on arresting anyone in the starport.
The XT line, like many legal fictions, is manipulated to the benefit of the parties, but most importantly the authorities. Under some conditions, may a ship be considered to have "exited" the starport? Certainly. What are those? Anyone's guess is as good as mine: conflict of laws is a nightmare with just a few dozen, very well-documented jurisdictions. When we talk about thousands of jurisdictions upon which less that a dozen pages have been written, then we need dozens of orders of magnitude more legal information to render anything more than a hazy guess.
Certain principles apply, less accurate than sweeping, and more useful than accurate:
1. All laws have exceptions.
2. Exceptions are often vague.
3. Most legal professionals do not even know all such exceptions in their own jurisdictions.
4. Most laymen don't fully understand even the most basic laws of their own jurisdiction; they have a fair understanding of some legal myths that are more true than false.
5. Most layman and professionals think they understand more law than they do.
As such, Refs should, IMHO, feel less compunction to understand and explain legal matters than they do to explain how the inside of a jump drive actually works. "You gauss rifle has been taken away, quite politely, by this Imperial Marine, citing some SPA reg you have never heard, and you have been given a claim stub, the back of which has a coupon for B1G1F tree kracken bites at Chandlers."
"Why?!?!?"
"Well, you don't really know. No one else seems to be carrying military longarms around, and you think you may have seen some sign, but you don't know where. The Marine is telling you to move along, and his smile is flagging. Some other members of his patrol are looking your way. Do you want to jump him?"
"What does the claim say?"
"Your weapon will be returned to your departing vessel, and the kracken bites are 600 grams a serving."
The genius of have a vague legal system of systems is that it is admirably quick. Law in the OTU is a blessedly vague abstraction, with only a couple of pavement markings. In my jurisdiction, where I practice on a professional basis, and in all the others in which I have taught law, legal decision-making never involves dice. A tragedy which has, though not an "expert," allowed me to make out a living within the minutia of the written law, and the far more important arcane, unwritten of the local courtroom work groups.
Keep it fuzzy! Pull out baffling details for chrome, but avoid logical explanation. The bigger the starport, the more regs, and the less understandable.
The regs will, IMTU which is my truest construction of the OTU, keep order but seek to avoid charges and formal procedures. A bit of "Be good, citizen!" [whack-whack], but no crimes, jails, or judges for simple mistakes. Thwarted wills, but no indictments. Sometimes, rarely, the locals creep in. Extradition for local offenses makes the improbably complex SPA regs look simple in comparison.
My Cr .02