Some people list rides as no drop (sometimes with re-grouping) but also make clear that you are expected to keep the advertised pace. The purpose it to, say, let a C rider know that if he/she shows up to a B+ ride and cannot keep the advertised pace, expect droppage. When that is made clear, I don't think you can fault a ride leader for leaving anyone behind. What is the leader supposed to do? Sacrifice his/her riding enjoyment because someone is not capable of keeping the advertised pace? That's a great way to discourage people from leading rides.
When I led club rides I would expressly state in the description that if you are far off the front or back you are on your own. I really do not like a person who "rides below their weight class." You get people who come on what are supposed to be more relaxed rides and then bust the pace. They clearly belong on a faster ride but likely don't want to risk getting dropped themselves. At least once I listed a ride where, to discourage pace busting, I expressly stated that the ride is intended to be true group ride and that if you might feel the need to fly off the front please pick a different ride.
And something you need to keep in mind about listed paces: Something like a strong wind can up that pace. Also, in our club, the posted average speed is not the maximum moving speed. It is an average. I (and many others) have always understood that to mean the average at the end of the ride. So if I list a 15 mph ride with both flat and hilly terrain, we may be doing 20 mph on the flats at times, 8 mph (or less) up hill, and 30 mph on descents.