There was absolutley no reason for Gamefreak to shiny lock the Ultra Beasts other than to be cunts. The whole "They appear in cutscenes lul" argument is incorrect.

The Nihilego is explicitly stated to be a different specimen than the one Lusamine fused with, and their are mutiple Buzzwole's and Pheromosa's, so one appearing in a cutscene is no reason to shiny lock both/all four.

Same applies for Necrozma and Zygarde, neither appear in the overworld or cutscenes.

Fuck you Gamefreak.