Most Japanese longblocks do have 30-60K miles, due to the peculiar tax structure on cars over there, but you're right; they may be low mileage by US standards, but rest assured they are HARD miles, especially the more popular engines (SR20, 1JZE, 2JZE, etc.)......

My mechanic buddy says that doing a straight swap is usually good for about a year before the engine fails, on average, unless you tear down and rebuild the thing upon receipt.