Micronutrients.
/We'll catch them when they come back for more food./@Rekesoft
These folks with FTL Volkswagons and food synthesizers - what would they need to return to civilization to get? Not food. Not clothes. Not fuel if they can collect hydrogen. Probably not religious services although that is an interesting idea.
Humans need micronutrients. They are on the vitamin bottle - cobalt, copper, zinc, etc. A matter synthesizer might be able to rearrange readily available carbon, hydrogen, nitrogen and oxygen and make fabric or food. Synthesizing an atom of manganese out of some other matter type requires more doing. Where do you find iodine in space? Recycling your wastes will reclaim micronutrients for a while but recycling is never 100%.
Of course if you don't want this, assert the matter replicator can make your manganese. But this premise presents another fine analogy to scurvy pirates - goiterous, ice-gnawing outlanders suffering the symptoms of gradual micronutrient deficiency and eventually being forced to make a run back to get more from the government monopoly, which doles them out sparingly.