I'd think it must be to sell laptops, not to sell Logic.

Now, do they need to do this to sell laptops, does it actually sell significant laptops, are there more people that buy a Mac laptop for Logic than who bought or would have a Mac laptop for Aperture? I dunno. It's also possible they miscalculated with Aperture. Or may be miscalculating now with Logic. Apple can make mistakes or behave irrationally too.

But it's gotta be to sell laptops. (Which is consistent with not charging for the upgrade; it's not about revenue from the software at all). And there are definitely at least some people buying a Mac laptop for Logic. (In a world with decreasing laptop sales in general, as many move phone/tablet only).

Now, with what you say about iPhones... I keep worrying that Apple will decide they don't really care about selling laptops after all someday...