No it's not unreasonable that they should break up into smaller groups that are easier to pass. And if they are backing up four or five cars, or even one car for several miles, it's not unreasonable that they should pull over and let the cars pass. Strictly speaking, convoys are illegal and big packs of bicycles staying together are acting as convoys.

No one has explained it completely but there's something under the surface going on here in our attitude about your question. Often someone gets behind a cyclist, and objectively it's only a few seconds or a minute at the very most before they can safely pass. But they are impatient and react out of proportion: honking, screaming, buzzing within inches, and sometimes worse. And you see and hear exaggerated claims about how much cyclists slow them down. You've seen those types acting that way with other cars I'm sure - imagine being on a bike instead of in a car when that happens. So that is a sore subject with cyclists.

As I read your scenario, that's not really the situation. You have a long stretch where it's really not safe to pass, and a group taking the road in the whole stretch without ever pulling over. As I understand it, that's pretty rare and I'd be upset about it also.