To add to you guy's answers, it's also to make people stop wasting their time SR'ing* for a shiny legendary.
Also, Victini is distribution-exclusive, so they just wanted to make sure everybody got one and nobody was dumb enough to keep SR'ing until the event was over, not getting a shiny, therefore not getting one at all.
^ They should do that to every event Pokemon that has a case similar to Victini.