If it's being called -- and marketed -- as a computer, then why is it OK to have a touch screen when a laptop (or iMac, for that matter) isn't?
Not trying to agitate anyone, just truly wondering.
(Yes, I know one it's a tablet... but so is a SurfacePro)
Use case.
The MacBook has an attached keyboard and trackpad, which is the primary mode of interaction. The UI is assumed to be optimised for keyboard and pointer input and a touchscreen is considered superfluous here.
Conversely, the iPad is simply a giant touchscreen. Yes, there is a keyboard and stylus accessory, but the touchscreen remains the sole mode of interaction for most users.
It's all about how Apple envisions you using their products.