The Girls und Panzer creators are done sitting by while fans sell derivative products based on the popular franchise.
— 「ガールズ＆パンツァー」公式アカウント (@garupan) June 7, 2018
They issued a statement on Twitter reminding fans that selling duplicates of official merch, derivative products like doujinshi, and self-produced goods is copyright infringement. The Twitter account urged fans to avoid purchasing unofficial products.
If you’re wondering how a major franchise can restrict the sale of unofficial goods and doujinshi, you’ll have to look at Comiket. At the creator’s request, Comiket can reject applications from Girls und Panzer doujin circles and forbid them from selling their goods. Shops in Akihabara that sell doujinshi can also receive notices to remove Girls und Panzer derivatives, restricting its existence only to the internet. It may not seem like a big deal, but most doujinshi and derivative goods are sold at conventions and specialty shops.
As a warning, a 33-year-old man was arrested in November for selling two stickers sheets bearing the school emblems from the anime. The Girls und Panzer rights owners don’t play around.