I would think it pretty obvious that DB knew that Radler was a generic term internationally - why else use the name unless they knew it would have some currency in NZ among those in the know or as a novelty for those not - they didn't just pull it out of their ass? I must admit that my beer knowledge wasn't what it was when Radler first came out and I probably hadn't heard of Radler before but I instantly knew it was was a style as it just isn't something a mainsteam NZ brewer would come up with as a unique brand name.
Anyway, it seems to me that the popularity / commonality of the term in NZ is irrelevant - Radler was a non trade-marked, historical beer style. To me, that simply means it shouldn't be allowed to be trade-marked in NZ. A company shouldn't be able to monopolise value from a term of this nature in a region where it may or may not have previously been common - however, I don't make the law and this may in fact be perfectly legal. In which case, the law is BS and should be changed.
The other thing is that if people had never come up with the BS idea of mixing good beer with Lemonade, then none of this would be an issue! (no offense to Green Man)
Interesting Wikipedia entry. To add to it, when I was in Chile several years ago they were drinking a mixture of Orange Fanta soda and light lager, called it a Fan-schop (schop being a term for beer if I remember correctly). You'd think it would taste like rubbish, but it was actually refreshing on a hot day.