Love Live! is one of the biggest anime franchises with a near global reach in terms of fans. While fans (especially in Japan) have a lot of opportunities to buy official merchandise, that doesn’t stop devout fans from creating fan merch at conventions.
Japan has a vibrant doujin community, which ranges from fan manga to various merchandise (like T-shirts and key chains). Well, the rights holders to the Love Live! franchise wants to put an end to the selling of unlicensed Love Live! goods.
The official Love Live! website was updated with a warning:
Unfortunately, despite repeated police investigations, Love Live! & Love Live! Sunshine!! merchandise that infringes upon copyright continues to be sold at events and on mail-order sites, as well as arcade prizes.
We will continue to deal strictly with copyright infringement, and continue our policy to not ignore complaints concerning these malicious actions.
Please take care not to purchase these unlicensed illegal goods. Thank you for continued support of Love Live! and Love Live! Sunshine!!.
Doujin goods based on pre-existing franchises that are sold at conventions (or elsewhere) is illegal under Japanese copyright laws, but only if the rights holders issue a complaint. Seeing as how the Love Live! holders have issued a formal complaint (which it seems like this isn’t the first time), fans might have a hard time find Love Live! doujin at non-official events.