It's illegal because the existing maps (the actual design of the maps) are protected works.The following are excerpts from the User Agreement that I'm sure you all read.

The computer software, artwork, music, and other components included in this product (collectively referred to as the "Software") are the copyrighted property of LucasArts Entertainment Company LLC and its licensors (collectively referred to as "LEC"). The Software is licensed (not sold) to you, and LEC owns all copyright, trade secret, patent and other proprietary rights in the Software. You may use the Software on a single computer. You may not: (1) copy (other than once for back-up purposes), distribute, rent, lease or sublicense all or any portion of the Software; (2) modify or prepare derivative works of the Software; (3) transmit the Software over a network, by telephone, or electronically using any means, except in the course of your network multi-player play of the Software over authorized networks; (4) engage in matchmaking for multi-player play over unauthorized networks; (5) design or distribute unauthorized levels; or (6) reverse engineer, decompile or disassemble the Software. You may transfer the Software, but only if the recipient agrees to accept the terms and conditions of this Agreement. If you transfer the Software, you must transfer all components and documentation and erase any copies residing on computer equipment. Your license is automatically terminated if you transfer the Software.aaaaaand:Rules Governing MODs and/or New Levels: MODs and/or new levels are data that modify, add to, or substitute for data in the Software, thus modifying, adding to, or replacing data provided by LEC in the Software, and may also include saved games (collectively, MODs and new levels are referred to herein as "MODs"). MODs may, or may not, be permitted by LEC, in its sole discretion, provided that you agree that the following conditions apply to your creation of any MODs: (1) MODs must work only with the retail version of the Software, and may not work with any demo or OEM versions of the Software. MODs may not be designed to be used as a stand-alone product.(2) MODs may not modify any .COM, .EXE, .DLL or other executable files in the Software. (3) MODs may not modify any .PK3 or other resource files in the Software. (4) MODs must not contain any illegal, scandalous, illicit, defamatory, libelous, or objectionable material (as may be determined by LEC in its sole discretion), or any material that infringes any trademarks, copyrights, protected works, publicity, proprietary, or other rights of any third party, or of LEC.(5) MODs must identify in every description file, on-line description, read-me, and in comments in the MOD code (if new code is added): (a) the name, address, and e-mail address of the level’s creators, and (b) the following disclaimer: "THIS LEVEL IS NOT MADE, DISTRIBUTED, OR SUPPORTED BY LUCASARTS ENTERTAINMENT COMPANY LLC. ELEMENTS TM & © LUCASARTS ENTERTAINMENT COMPANY LLC AND/OR ITS LICENSORS."(6) MODs may not be sold, bartered, or distributed on a stand alone basis or with any other product for which any charge is made, including, without limitation, renting, leasing, retail sales and/or on-line electronic distribution (other than incidental charges for time spent on-line), but rather must be distributed free of charge. You agree not to solicit, initiate or encourage any proposal or offer from any person or entity to create any MODs for commercial distribution. You agree to promptly inform LEC in writing in the event you receive any such proposal or offer.(7) By distributing or permitting the distribution of any MODs, all creators or owners of any trademark, copyright, or other right, title or interest therein grant to LEC an irrevocable, perpetual, royalty-free, sublicensable right to distribute the MOD by any means (whether now known or hereafter invented), and to create and distribute by any means (whether now known or hereafter invented) derivative works thereof, and to charge for the distribution of such MOD or such derivative work, with no obligation to account to any creators or owners of the MOD in any manner.The second part is debatable, since you can't actually modify Bespin. You'd have to recreate the entire level with your modifications, but that would violate the first part.