It's not a perfect world, obviously, but the person who seals the trailer is nonetheless responsible for checking load security. If a deck collapses in the nose of a trailer behind several pieces of freight that made seeing the deck impossible, you can't really be held accountable for that. The idea with an enclosed van trailer of any kind (dry, heated, reefer) is that the door(s) are the path of least resistance and should be safeguarded. Will anything fall against the door, causing a risk to the person who opens it? Might something break loose and possibly break open the door?
It's like a circle check. You are supposed to try your best. A half-loaded smooth wall trailer doesn't leave you options unless you have a rubber footed expanding load bar, for example. Likewise, a trailer sealed by the shipper is impossible to check.
That being said, I deal with a lot of time-sensitive linehaul work, including some special commodities. The only time I've had a pre-sealed trailer was if another driver picked it up from the shipper, with the sole exception of computer equipment shipments. And I cross the border, something that mandates high security seals. Most shippers, in my experience, don't like being responsible for sealing the trailer themselves unless there's a high risk of theft.
My trailer is pulled from the dock and hooked by the hostler and put on ready line.
The dispatcher hands my bills and asks, "would you mind putting this seal on your trailer?"
"You feel I am responsible for this?" you gotta be kiddin.