The pattern you can see repeated over and over in Pali Suttas, is Buddha saying: "there is a case" when someone does X - then later it can be expected the result Y will follow. Much of Buddha's guidance takes form of such statements, "this is how things usually work out".
In some other suttas Buddha says "having a belief Z will help towards Y because it will motivate the person to do X".
-- These two are what you call "descriptive", right?
In some other suttas Buddha simply talks about wholesome/unwholesome behavior without connecting it to goals.
-- These seemingly appear to be what you call "normative" - however I think they are really just implicitly descriptive. It is just that the goals and beliefs are implied, depending on the type of the audience. He implies either the layman goal of happy/enjoyable living or the recluse's goal of attaining the final Enlightenment/Liberation/Nirvana.
Finally, in some other sutta Buddha says "ABC is a valid goal too, but Y is the highest goal, the ultimate goal".
-- This clearly looks like a normative kind of statement.
So, it looks like the Buddhist model of rationality is almost completely descriptive with the exception of the (normative) superiority of Enlightenment over all other goals. Besides the Enlightenment, which seems to be the didactic implant, most of the rest of Buddhism is highly descriptive:
Moggallana and Sariputta stayed with Sanjaya for some years, leading the life of wandering ascetics, but they were not really satisfied with what they were learning from their teacher. After a while they decided to split up and each go their own way in search for truth, promising that the first to find it should tell the other. One day, as Sariputta was walking through Rajagaha, he saw a monk and was deeply impressed by the grace and poise with which he moved and the calm happy expression on his face. The monk happened to be Assaji, one of the [first five] Buddha's disciples. Sariputta asked him:
"Who is your teacher?" and Assaji replied, "Friend, there is a great ascetic, a son of the Sakyans, who went forth from the Sakyan clan. It is because of this Lord that I have gone forth. This Lord is my teacher, I accept this Lord's Dharma."
"What doctrine does your teacher teach? What does he point to?"
"Friend, I am a beginner, I have only just gone forth, I am new in this Dharma and discipline. I cannot teach the Dharma in full, but I will tell you its essence."
"So be it, your reverence, tell me little or tell me much, but either way give me its essence, I just want the essence. There is no need for great elaboration."
So Assaji said: "Those things that proceed from a cause, of those things the Tathagata has told the cause. And that which is their stopping, of that the great recluse also has a doctrine."
