> To me a promo not just in manga but in most areas is meant to be a taste or a tease to get you to get more of whatever product.
I bolded the part where you are wrong. You're judging "promotional" (promo is simple contraction) from western perspective. We'll give you one bigmac for free and you are our slave for life.
Japenese perspective is different. For example, do you know that in most cases anime by itself are barely profitable or studio can even lose money?
But when studio produce anime profit from anime is not the target. Anime is promotional product. They will lose money on anime but will make fortune from LN/manga sale spikes, from merchandise, from OST/singles, etc.
That's why in every thread for promo tag I reply with Any non-original manga is promo manga. In many cases manga is less detailed and much more fastpaced - manga is promotional product for LN (better, more detailed product).