What is the meaning of "Mah ha'Bamah"?
Radak: When I saw them offering to idolatry everywhere, and I designated a special place for My name, and they did the opposite of My command in two ways. (a) They did everywhere, and even for My sake. (b) They offered to other gods. Mah ha'Bamah - who commanded you to make a tall Bamah? I commanded you to make "Mizbach Adamah" (Shemos 20:21). You learn from the Goyim to make a tall Mizbe'ach called a Bamah due to its height. Or, it is on tall mountains. Bamah is an expression of height, like "Yarkivehu Al Bamasi Aretz" (Devarim 32:13), "E'eleh Al Bamasi Av" (Yeshayah 14:14). Since they learned from the Goyim to make a Bamah, they learned from them also to do it everywhere, and to serve other gods with it.
Malbim (28-29): In the Torah, Sefer Yehudah and Shoftim, we never find a Bamah to Hashem, only for idolatry. It says "Mizbach Adamah Ta'aseh Li" (Shemos 20:21), and warns not to offer in every place. It forbids planting an Asherah or any tree by the Mizbe'ach, like the rite of idolaters. When they came to the land they began to offer on Bamos. I protested immediately, and said 'what is this Bamah? It is proper for idolaters, but not for you!' In the days of Shmuel, after Shilo was destroyed, they began to offer to Hashem on Bamos. This remained even after the Bayis was built (and Bamos were forbidden), until Chizkiyah removed them.
Why does it say "Asher Atem ha'Ba'im Sham"?
Radak: This addresses those who serve on the Bamos. You come to there, and not those who guard My Mitzvos. The latter come only to the place special for Me.
What is the significance of calling it a Bamah?
Rashi: This is an expression of disgrace - ba'Meh Hi Nechsheves (what is it considered)?
Radak: Based on how people speak, even what was made for Me became called a Bamah. It says that Shmuel will ascend "ha'Bamasah" (Shmuel I, 9:13). Shlomo and the nation went "la'Bamah Asher b'Giv'on" (Divrei ha'Yamim II, 1:3). It says "Rak ha'Bamos Lo Saru" (Melachim II, 12:4), based on the habit that they used to be for idolatry. They did not cease making them even in the days of the kings who did ha'Yashar b'Einei Hashem; they made them for My sake.
Malbim: Even though I protested and said that it is not for you, you called it a Bamah, and it became rooted in you until today.
