Who says they don't? AFAIK no one has asked Northshore to comply a car imported through someone else. They might do exactly what you suggest.
It's also their reputation though, if it was my business I wouldn't want someone importing a dog then telling all their mates that my business did the compliance, or worse, "I got it through XX".
Remember that $4k profit on the car also covers the time necessary to find a nice, clean car, verify everything, service it, etc.
I bought my car from Northshore, and yes I probably could have saved some money buying direct from Japan, but in the end it suited me at the time. They had exactly what I wanted, in stock, at what I thought was a reasonable price. Ben bent over backwards to help me out, and I was a pain in the arse, having never bought a car from interstate before. It was easy for me, and I was prepared to pay for the convenience. At the end of the day my time is worth something, and I couldn't be happier with the result!