Hi Norman, while it doesn't go into specifics, I would have to presume (A dangerous passtime, I know.) that at least some of the cargo was volitile enough that the only place the crew could smoke was on the weather decks. If you check out Earnest Gill's testimony on day 8 of the U.S. Senate Inquiry, you'll see in his appended affidavit the following quote;
"I turned in, but could not sleep. In half an hour I turned out, thinking to smoke a cigarette. Because of the cargo, I could not smoke 'tween decks, so I went on deck again."
It's not unknown for merchent vessels to travel without cargo, but for the reasons you just mentioned, shipping lines don't make a habit of it. They'ed go broke if they did.
Cordially,
Michael H. Standart