Why does it say "Ach"?
Rashi #1: Only in this, Shlomo did improperly. Yaravam rebuked him for this. Your father left [the Milo] open for Olei Regalim, and you built it for Bas Pharaoh's slaves?!
Rashi #2: The cities mentioned above were needed for storage, chariots, horsemen and grandeur. He did not build the Milo for grandeur, for his father left it for Olei Regalim to spread tents there. Rather, Bas Pharaoh ascended to her house, and the Milo was near her house - then Shlomo built the Milo.
Why did Bas Pharaoh leave Ir David?
Rashi: Initially she was in Ir David. In Divrei ha'Yamim II, 8:11 it explains that Shlomo did not want to have a wife in Ir David, for it is Kadosh; the Aron came to there.
For what did he build the Milo?
Rashi: He made buildings for her male and female servants.
Radak: He built it for a house for Bas Pharaoh.
Here it says that Bas Pharaoh ascended [by herself]. In Divrei ha'Yamim II, 8:11 it says that he made her go up!
Malbim: Shlomo did not want her to be in Beis David, so he caused her to ascend. He did not tell her so, rather, that he builds a house just for her, for her honor; she went willingly.
