I cite statement by Daiyuu Nobori, from GitHub issue pertaining to this.
The text "USE ONLY IN JAPAN. DO NOT USE IT IN OTHER COUNTRIES." in the readme file is just our statement for warning and disclaimer. This is not a restriction under the license. Since SoftEther VPN has been licensed under GPLv2, we do not impose any additional restrictions which have contradiction with the license. We must put our disclaimer, otherwise we have a risk to be sued or sentenced by any court of any country in the rest of the world. In order to avoid or eliminate such a risk, we have put this "USE ONLY IN JAPAN" statement in the readme file.
However, we are planning to replace this disclaimer statement to reflect our clear intention in the next update of the source code. The new statement will be as the following:
Use only in Japan. Do not use this software in another country unless you have a confirmation that this software does not violate any criminal laws or civil rights in that particular country. Using this software in other countries is completely at your own risk. The SoftEther VPN Project has developed and distributed this software to comply only with the Japanese laws and existing civil rights including patents which are subjects apply in Japan. Other countries' laws or civil rights are none of our concerns nor responsibilities. We have never investigated any criminal regulations, civil laws or intellectual property rights including patents in any of other 200+ countries and territories. By nature, there are 200+ regions in the World, with different laws. It is impossible to verify every countries' laws, regulations and civil rights to make the software comply with all countries' laws by the project. Even if you will be sued by a private entity or be damaged by a public servant in your country, the developers of this software will never be liable to recover or compensate such damages, criminal or civil responsibilities. Note that this line is not license restriction but just a statement for warning and disclaimer.
License further contains the following statement:
THIS SOFTWARE IS DEVELOPED IN JAPAN, AND DISTRIBUTED FROM JAPAN, UNDER
JAPANESE LAWS. YOU MUST AGREE IN ADVANCE TO USE, COPY, MODIFY, MERGE, PUBLISH,
DISTRIBUTE, SUBLICENSE, AND/OR SELL COPIES OF THIS SOFTWARE, THAT ANY
JURIDICAL DISPUTES WHICH ARE CONCERNED TO THIS SOFTWARE OR ITS CONTENTS,
AGAINST US (SOFTETHER PROJECT, SOFTETHER CORPORATION, DAIYUU NOBORI OR OTHER
SUPPLIERS), OR ANY JURIDICAL DISPUTES AGAINST US WHICH ARE CAUSED BY ANY KIND
OF USING, COPYING, MODIFYING, MERGING, PUBLISHING, DISTRIBUTING, SUBLICENSING,
AND/OR SELLING COPIES OF THIS SOFTWARE SHALL BE REGARDED AS BE CONSTRUED AND
CONTROLLED BY JAPANESE LAWS, AND YOU MUST FURTHER CONSENT TO EXCLUSIVE
JURISDICTION AND VENUE IN THE COURTS SITTING IN TOKYO, JAPAN. YOU MUST WAIVE
ALL DEFENSES OF LACK OF PERSONAL JURISDICTION AND FORUM NON CONVENIENS.
PROCESS MAY BE SERVED ON EITHER PARTY IN THE MANNER AUTHORIZED BY APPLICABLE
LAW OR COURT RULE.
USE ONLY IN JAPAN. DO NOT USE THIS SOFTWARE IN ANOTHER COUNTRY UNLESS YOU HAVE
A CONFIRMATION THAT THIS SOFTWARE DOES NOT VIOLATE ANY CRIMINAL LAWS OR CIVIL
RIGHTS IN THAT PARTICULAR COUNTRY. USING THIS SOFTWARE IN OTHER COUNTRIES IS
COMPLETELY AT YOUR OWN RISK. THE SOFTETHER VPN PROJECT HAS DEVELOPED AND
DISTRIBUTED THIS SOFTWARE TO COMPLY ONLY WITH THE JAPANESE LAWS AND EXISTING
CIVIL RIGHTS INCLUDING PATENTS WHICH ARE SUBJECTS APPLY IN JAPAN. OTHER
COUNTRIES' LAWS OR CIVIL RIGHTS ARE NONE OF OUR CONCERNS NOR RESPONSIBILITIES.
WE HAVE NEVER INVESTIGATED ANY CRIMINAL REGULATIONS, CIVIL LAWS OR
INTELLECTUAL PROPERTY RIGHTS INCLUDING PATENTS IN ANY OF OTHER 200+ COUNTRIES
AND TERRITORIES. BY NATURE, THERE ARE 200+ REGIONS IN THE WORLD, WITH
DIFFERENT LAWS. IT IS IMPOSSIBLE TO VERIFY EVERY COUNTRIES' LAWS, REGULATIONS
AND CIVIL RIGHTS TO MAKE THE SOFTWARE COMPLY WITH ALL COUNTRIES' LAWS BY THE
PROJECT. EVEN IF YOU WILL BE SUED BY A PRIVATE ENTITY OR BE DAMAGED BY A
PUBLIC SERVANT IN YOUR COUNTRY, THE DEVELOPERS OF THIS SOFTWARE WILL NEVER BE
LIABLE TO RECOVER OR COMPENSATE SUCH DAMAGES, CRIMINAL OR CIVIL
RESPONSIBILITIES. NOTE THAT THIS LINE IS NOT LICENSE RESTRICTION BUT JUST A
STATEMENT FOR WARNING AND DISCLAIMER.
Essentially, developer of the software is saying, "we don't care if you use it outside of Japan, but we have not investigated usability outside of Japan, and if you do get into trouble by using it abroad, don't expect us to help you." Talk to your lawyer if you are really worried.
Please do note, just as any GNU Public License (GPL) licensed software, the software is provided "WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE."