It's not quite as simple as that. The development of Clang is being funded by Apple. They need a BSD license so that they have the freedom to make further modifications down the line (without leaving them open). Yes, I'm a GPL advocate. No, I don't agree with Apple's ideology. But it's the case anyway.
I doubt that's accurate view of their motivation - although neither of us can prove it either way. But judging on past form they don't seem to have held back their Clang modifications so far, why would they want to start doing so later?
I think it is more likely that they are worried about the patent grant implications of GPL 3. A lot of corporations are, rightly or wrongly. Certainly in the company I work for, the legal department are paranoid about the idea that one of our contributions to a GPL 3 project might be picked up and (legally) included in unrelated projects, which doesn't necessarily need to be software products, and thus we might be deemed to have granted a license to all our patents to a hardware competitor. Now me and the lawyers can disagree on how likely that is to happen in reality, but the final words is that it's way easier for me to get corporate approval to send changes upstream to a BSD licensed project than a GPL licensed one. I suspect something similar is going on at Apple, and that backing the BSD-licensed clang project is enabling them to be a better participant in the open source community, not a worse one.