Exactly. And the individual worlds are responsible for setting and enforcing their own restrictions on trade, in- and outbound.
This does lead to an interesting potential quirk in the interlocking roles of planets and Imperium. A planet may reasonably insist that all traffic arriving on the surface must land at a designated starport facility. This makes proper customs inspections possible. But Traveller ships are easily capable of landing effectively anywhere, so without some force to back up that rule, it's meaningless. Given the Imperial interest in supporting the sovereignty of member worlds, I can easily picture a world beset by smugglers requesting and receiving enforcement assistance from the Imperial Navy. Just knowing that this *can* happen, without warning. would help keep the Imperium-wide level of outback-landing smuggling operations under control.
And of course, this leads to some highly, ah, entertaining situations to inflict on a crew of somewhat ethically-challenged merchants. E.g., you've fallen into a happy routine of picking up a hold full of laser rifles on Gemmet, jumping over to Lysander, and landing in a meadow near the Johnson Mountains on the southern continent, in the heart of Free Lysander Movement territory. Lysander is a low tech, cash-strapped world, so there's no worry about being intercepted, or even detected half the time. So one day you drop off your shipment, collect the usual payment in cash and cargo, hop in your ship, and climb out of the atmosphere. At which point every radar and lidar sensor telltale on the board turns red, and a voice crackles over the radio: "The is the Imperial Navy light cruiser Ragnarok. You are ordered to cut your engines and prepare to be boarded. Confirm your compliance on this frequency. You have five minutes to respond. Signal repeats. This is the Imperial Navy light cruiser Ragnarok..."