Hi Takumi,
Planting a pine, or indeed any tree in the ground in general, allows it to grow bigger, quicker. This is because the roots have as much room as they want/need to draw nutrients and water from the surrounding soil. The more root mass, the more top growth the tree will exhibit. Obviously, the more suitable the soil, the better the tree will grow.
They will not become rootbound, unless there physically is something stopping the roots from spreading in a certain direction IE... if your growing bed had solid rock or concrete directly under it or building foundations at the sides etc. Trees only get root bound in pots because the sides and base prevent the roots from spreading any further, the same would need to happen in the ground in order for the roots to become "bound".
Once the desired trunk girth is achieved then yes the tree can be removed from the ground and placed in a training pot. The tree will need time to adjust, a whole year, sometimes longer before any styling should be attempted.
Try reading some of Vance Woods threads on Pines as he is extremely knowledgable about them. Also
Bonsaikc has great knowledge of Japanese Black Pines that I am aware of.
All the best,
Aaron