Chosen Place

The possuk (12:4-6) says לֹא־תַֽעֲשׂ֣וּן כֵּ֔ן לה' אלוקיכם כִּ֠י אִֽם־אֶל־הַמָּק֞וֹם אֲשֶׁר־יִבְחַ֨ר י״י֤ אֱלֹֽקיכם מִכָּל־שִׁבְטֵיכֶ֔ם לָשׂ֥וּם אֶת־שְׁמ֖וֹ שָׁ֑ם לְשִׁכְנ֥וֹ תִדְרְשׁ֖וּ  וּבָ֥אתָ שָּֽׁמָּה וַֽהֲבֵאתֶ֣ם שָׁ֗מָּה עֹלֹֽתֵיכֶם֙ וְזִבְחֵיכֶ֔ם.  Rashi says לשכנו תדרשו: זה משכן שילה:  This possuk tells us that when the Mishkan was in Shilo, one wasn't allowed to sacrifice on a bammah.  In possuk 11 it says הָיָ֣ה הַמָּק֗וֹם אֲשֶׁר־יִבְחַר֩ יְהֹוָ֨ה אֱלֹֽקיכם בּוֹ֙ לְשַׁכֵּ֤ן שְׁמוֹ֙ שָׁ֔ם שָׁ֣מָּה תָבִ֔יאוּ אֵ֛ת כָּל־אֲשֶׁ֥ר אָֽנֹכִ֖י מְצַוֶּ֣ה אֶתְכֶ֑ם.  Rashi explains that possuk is referring to the Beis Hamikdash; when there is a Beis Hamikdash, there also is an issur bammos.

There is a difference in the obligation to bring a korban to Shilo vs. the obligation to bring to the Beis Hamikdash.  The difference between Shilo and the Beis Hamikdash is that the issur bammos in Shilo is a result from לֹא־תַֽעֲשׂ֣וּן כֵּ֔ן.  Therefore, there had to be a place to sacrifice and that is Shilo.  The place is chosen by Hashem לָשׂ֥וּם אֶת־שְׁמ֖וֹ שָׁ֑ם, but the place itself isn't inherently the "chosen place."  Hence, the  possuk puts the focus on the person bringing the korban to the place, וַֽהֲבֵאתֶ֣ם שָׁ֗מָּה.  It isn't the place that demands the korban be offered there, it is the place that happens to be designated to bring everyone together.  The second possuk focuses on the place, שָׁ֣מָּה תָבִ֔יאוּ, which forces a person to bring the korban there.  It is in the place of the Beis Hamikdash that is 'הַמָּק֗וֹם אֲשֶׁר־יִבְחַר֩ ה, the actual place is desired to be the place of the Mikdash and the issur bammos is an outgrowth from there.  It is the kedusha of the place that causes the issur bammos.  Therefore, the possuk mentions first that the place is "chosen" and the issur bammos is the outgrowth from that (based upon Likutay sichos volume 24.)
 
The Mishna in Megillah (10a) and Zevachim (112a) says that once the Beis Hamikdash was built its no longer permitted to sacrifice on a bammah.  The Mishna says that the intent of the possuk, כִּ֥י לֹא־בָאתֶ֖ם עַד־עָ֑תָּה אֶל־הַמְּנוּחָה֙ וְאֶל־הַֽנַּחֲלָ֔ה, menucha refers to the Beis Hamikdash that once it was built one can no longer use a bammah.  The Gemorah in Megillah assumes this is true only according to the opinion that קדושה ראשונה קדושה לעתיד לבא, however, if the קדושה doesn't continue after the Beis Hamikdash is destroyed, then one can sacrifice on a bammah nowadays.  Rashi (Zevachim 119a) follows this approach and says the Mishna is dependent on the machlokes tannaim.  However, Tosfos (Megillah, Zevachim) understands according to the conclusion of the Gemorah that everyone agrees even after the destruction of the Mikdash the bammos are prohibited.

Why does Tosfos holds even if there is no more kedusha that the issur bammos still applies?  The Achronim explain that even though there isn't kedusha to be able to bring a korban, however, it still is the 'מָּק֗וֹם אֲשֶׁר־יִבְחַר֩ ה.  It isn't the kedusha of the Mikdash that causes the issur bammos, it is sacrificing outside the "chosen place." (See Toras Hakodesh siman 2.)