It's pretty much common sense. It's simply not practical to rely entirely on smuggling-only transports to get illegal goods around, given the risk of having such vessels get caught by authorities. Hiding illegal goods amidst an otherwise-legal shipment is considerably more reliable, since then you pretty much just need to be lucky enough for your container to not be one they decide to look closely at. That's not to presume that the companies shipping legitimate stuff are complicit in the smuggling, since there's (generally) no reason to suspect that, but it does happen.
Yes but he said that they only get shipped like that instead of smuggling too. ---