I can't speak to the decisions any manufacturer may make. There is no requirement that the manufacturer 'list' any part of a production run.
Indeed, it's fairly comon for a manufacturer to affix the UL sticker to only part of a production run, or only upon specific request by the customer. UL's position is that no lable = no listing. With the esception of very small items, the UL mark must be on each individual completed item. Small parts can have the marking on the packaging.
Why would a company make perfectly fine items, and not place the UL lable on them? In a word, cost. Firms are typically charged for each lable used; one might even argue that UL owns the lable itself.
You can be sure that UL listing for ladders is a particularly expensive listing. This would be the direct result of our unpredictable, lawyer-driven legal system. If there's ever a suit - and there often are - involving a ladder in the least manner, UL is sure to be named in that suit. While UL is invariably excluded from the suit early in the proceedings, it costs money to respond.
As for your particular ladders, there are other factors that may have come into play. The retailer may have desired a 'special' price. They may have been made elsewhere, in a non-inspected plant, and re-labled on importation. The ladders may have originated with the (now defunct) Keller ladder branch of Werner, over which there were some legal / PR wrangling. Or, the UL lable may have been printed as part of another lable that has since been removed. (Before long, I expect there to be a law requiring all ladders to be at least 8ft. long, just so there's room for all the lables!)
Also, keep in mind that UL does not have as strong a presence in every market; their name may not add much value to a ladder. A classic similar example is the electrical rating of hard-hats; few, if any, have a UL tag on them.
