It seems a bit strange that the "registrar" would impose this restriction, unless there is some local government restriction (as you seem to imply)? Although I'm not sure why that would be?
ns1.twnic.net.tw would seem to be the NAMESERVER of the register (TWNIC), not the registrar (for which there only appears to be 13 officially listed). As far as I know, TWNIC (the register) does not impose such a restriction.
In fact, TWNIC gives specific instructions for updating the nameservers on their site (See Domain FAQs under "Q25: How to set up the DNS for the English Domain Name?"). But this is only for when the domain is registered directly with TWNIC - which does not seem to be possible these days:
Since March 1, 2001, TWNIC has stopped allow itself to sign up new domain names directly, instead allowing new registration through its contracted reseller registrars.