^ Making all legendaries shiny-locked just because you can't obtain them by yourself... Sounds pretty selfish to me.
Also, there's no way next movie's Genesect event would not be shiny, really.
I don't really have any definite ideas or theories about Reshiram and Zekrom, but Victini has something in common with Entralink Pokemon (which are also shiny-locked); it's injected into your game and placed in a location as an interactive Pokemon. Which makes me believe that any interactive Pokemon either spawned through outside injection or an event like Reshirams/Zekroms were either chosen to all be shiny-locked, or there is a coding issue with the way their PIDs are generated that prevents them from being shiny. Hidden Hollow Pokemon also are shiny-locked and are randomly generated interactive Pokemon.
Stationary legendaries are also interactive, but their spawns are different. Comparing them is like comparing a Pokemon fished from regular water and one from rippling pools.
Victini isn't injected into your game.
You merely receive the Liberty Pass, which is used to access Liberty Island, all of which was hardcoded into the game from the start.