I want to applaud GitHub for doing this in a peaceful manner. They have every right to ask this. I think having all project develop their own UI style will allow for more innovation to happen. At GitLab we are greatly inspired by GitHub but over time the GitLab became more distinctive. And that allowed a trickle of innovation to flow back, for example protected branches.
And notice they said its a good first start. This will get similar to Apple having issues with corner's curve of Samsung devices... never enough curved to actually not remind of Apple's products.
Bad move on Github part, but I'm happy with Btibucket anyways.