Some of these ANSIR style names don't make a lot of sense relative to other systems. The Idealist in this system is one of the Practicals. To me that name correlates with an MBTI Sensor, for example, the ESTJ. The ESTJ is usually a model citizen, and pillar of the community. He or she takes their commitments seriously, and follows their own standards of "good citizenship" to the letter.
Ideal: "Conforming to an ultimate standard of perfection or excellence."
"Model" citizen, high standard of behavior. Does "Idealist" make more sense now?
The name "Idealist" is commonly used to describe someone who pursues a more "progressive" agenda. But in this case it describes someone much more conservative, someone who maintains the ideals of the community around him/her. This is not some idealist who strives to change the world according to a mere idea. This Idealist is a conformist to a social ideal that already exists.