It is the responsibility of the consignor to declare what is being shipped (the consignor is whomever pays for shipping). The shipper is not responsible in this case. However, with international shipments, things are different. The US government is trying to get companies to inspect their cargo before leaving foreign ports and coming to the US, so in cases where shippers are required to check their cargo, they are obviously responsible for the contents of their containers.
Private carriers are always responsible, as they own the transportation which they ship their goods on (e.g. Walmart would be responsible for whatever is shipped on their private fleet of trucks).
I don't think carriers are liable. I think it's "common carrier status" - same as phone/Internet companies not being liable for illegal activity conducted over their networks.
You're right - but I'd imagine it's different for contract carriers, who have a much different (closer) relationship with their customers.
Common Carriers: Must serve any and all customers until capacity is filled.
Contract Carriers: Serve small amount of customers and can refuse outside customers.
But in all cases, the shipper would have to deal with the consequences of shipping illegal goods. Obviously incidents can tarnish reputations and cause legal problems, so for some companies it is in their best interest to know exactly what is being shipped.
I like this stuff, it's what I'm majoring in at school. I've learned some of the law and liabilities of carriers but not all the specifics, my law class is next year.
