Quote from Liesha;1293268:
It's against Nexon Japan EUA.
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(7) 日本国外ã®ãƒ‰ãƒ¡ã‚¤ãƒ³ã®é›»åメールアドレスを用ã„ã¦ç¬¬3æ¡ç¬¬1é …ã«å®šã‚る手続を行ã£ãŸå ´åˆ
Article 4 (registration of disapproval and reserves)
(7) If you are using an e-mail address of the Japanese foreign domain was conducted with the procedure set forth in Article 3 Paragraph 1
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当社ã¯ã€æœ¬ã‚µãƒ¼ãƒ“スãŠã‚ˆã³æœ¬ã‚½ãƒ•トウェアã¸ã®æŽ¥ç¶šã‚’ã€æ—¥æœ¬å›½å†…ã‹ã‚‰ã®ã‚‚ã®ã€ã¾ãŸã¯æ—¥æœ¬èªžã®ã‚ªãƒšãƒ¬ãƒ¼ãƒ†ã‚£ãƒ³ã‚°ã‚·ã‚¹ãƒ†ãƒ ã«ã‚ˆã‚‹ã‚‚ã®ã«é™ã‚‹ã“ã¨ãŒã§ãã‚‹ã‚‚ã®ã¨ã—ã¾ã™ã€‚
Article 25 (connection limit)
The Company assumes that the connection to the Service, and this software, it can be limited to those by those from Japan or a Japanese operating system,.
(just rammed it thru Google translate without editing)
If we're totally technical here
It's not illegal to play JP Mabi from NA.
It's however illegal to register an Account using a Foreign E-mail or bypass an IP-Block.
So if you have a JP E-mail and somehow get a Japanese IP without a Proxy you're not doing anything wrong.... It's just impossible to sit in NA and get a Japanese IP without a Proxy.
@OP
In Short: The act you ask about isn't Illegal, the way to achieve it is.