This is the kind of thing that is really dangerous to the abortion power base. Explicitly accurate terms like “killing babies” are an anathema to them. “Choice”, “freedom” are “tissue” are the only accepted words in their abortion lexicon. They’ll deal with older pro-life people, but teen sensations have the power to influence millions of kids for the rest of their long lives.
I've always laughed my a$$ off at their euphemistic phrases. Nobody thought it was funny when the phrase, “final solution” was created, yet it is EXACTLY in the same vein. "Great" minds apparently think together!
Yes, I think you're exactly right. To an adult, Bieber's just another mindless pop-star. But, to his base, he's a star - a star who probably wields at least some influence.
I suspect that people who early on in their life develop a sense that babies in the womb are just that - babies, generally don't move into the militant "pro-choice" camp later on. To the abortionists, this is a war for hearts and minds, and Bieber presents a frontal assault on their position with his very simple phrase, "it's like killing babies". They can't let that stand.