It seems like you know what a Light Novel is, so I don't know how much needs to be explained since it's kind of self-explanatory. LN promos are just what people call manga that were created to drive sales of the LN they were adapted from (the same happens for WNs).
They don't have a reason to speed up the resolution + wrap things up in a single chapter (even if they could), because what makes a good promo is something that leaves people wanting more. If you give people a suitable conclusion, they're not as likely to go look for +buy the real ending, because they're already satisfied. Some people might be like you and be more likely to buy the LN just to support the author if the manga has a good ending, but that's a minority compared to the people who'll buy something because they "need" to see what happens next.
(fwiw, I don't really know how useful a "LN promo" tag would be anyway, because it's not like these manga announce themselves as them. They just end and you take a guess as to whether it was an unplanned end (axe) or a planned end (promo), but nobody really knows it's ending until the end, when 90% of readers are already invested)
