Copyright Infringement? LibreBoard – Trello

See also https://news.ycombinator.com/item?id=8936701 (back when it was first presented and this happened)

Seems like they were kind of surprised by the fact that people actually looked at their toy as a serious thing. If you just play with something yourself it isn't an issue if you use others' assets/code or not, but as soon as you publish it that doesn't fly anymore, and fixing that didn't happen when they made that step.