In fact, it is under its own custom license, which is derived from BSD:
> Copyright (c) 2004-2012, Aegisub Project
> All rights reserved.
>
> Redistribution and use in source and binary forms, with or without
> modification, are permitted provided that the following conditions are met:
>
> * Redistributions of source code must retain the above copyright notice,
> this list of conditions and the following disclaimer.
> * Redistributions in binary form must reproduce the above copyright notice,
> this list of conditions and the following disclaimer in the documentation
> and/or other materials provided with the distribution.
> * Neither the name of the Aegisub Group nor the names of its contributors
> may be used to endorse or promote products derived from this software
> without specific prior written permission.
>
> THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
> AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
> LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
> CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
> SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
> INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
> CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
> ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
> POSSIBILITY OF SUCH DAMAGE.
> --- END AEGISUB LICENSE --
>
> Any file that does not have a license in the header is covered by the above
> license with exception to any directories that contain a license file, or are
> covered by another license in the file itself.
>
> The following directories and file are covered by their respective licenses as
> follows:
>
> src/gl/
> - MIT license. See src/gl/glext.h
>
> src/MatroskaParser.(c|h)
> - Licensed to BSDL with permission from the author.
>
> universalchardet/
> - MPL 1.1
Vítor Galvão