No problem. It`s something that is easily mistaken.
In my 9th year of marriage here in Japan, and have an official alias using the kanji surname... But unfortunately it`s just an "alias".

That is so strange >_>

I can't even think of any good reason as to why you are not allowed to write your name in Kanji except to single you out as a foreigner.

It does make sense in it`s own way if you actually know the rules behind it.

Basically, as you are not a Japanese citizen, you do not change your name in Japan. You register a name change with your native country. If your native country does not use kanji, you obviously cannot register a name in kanji with them. Romaji is only an interpretation of a Japanese name, not the name itself.
So, when you present a name that has been registered in your own country, in the roman alphabet, it is not a Japanese name... No matter what you derived it from. Legally, they are obligated to interpret it into Japanese via katakana, just as someone living in an English speaking country is obligated to have their name written in the roman alphabet even if their name is originally written in a completely different character set.

But, just as you can use the original written form of your name as a signature regardless of the country you are living in - you can use the kanji form of your name as an alias/signature in Japan.

There are lots of names that exist in both Japanese and English - it would really be a mess if they were simply allowed to pop in any random kanji for them.

I am still not sure about business cards or in my case calling card. These are not official and also make it easy for Japanese to understand the name if you use kanji.
My instance of given name is Marc and I have selected with help the following kanji... 真亜久 which I now am using on my calling card.

I have had quite a few of these cards marvelled at and taken with much gusto. I think I know why but I am keeping quiet.

Business cards are not official, so I would see absolutely no problems arising with using kanji on them.
I disagree though that it would make it easier to understand your name... Maybe if you`re using a Japanese surname, yes. But a foreign first name in kanji is far more confusing than simply having it written out in katakana.