1)
Why does it say "ha'Har Shomron"?
1.
Radak: This is like ha'Am Yisrael (refer to 16:21:2:1).
2.
Targum Yonasan: Keraka. Radak - he holds that it was initially a big city, but Amri strengthened it and called it Shomron.
2)
Why does it mention the reason for the name Shomron?
1.
Malbim: This teaches the difference between Yerushalayim, which was called to recall Yir'ah and Sheleimus, and Shomron, which was called on the name of a Rasha!