Not yet (see below).
Yes. You can download the trial version, then buy it online without any physical media being involved.
Yes, if you are a paid member of the Mac developer program. It just isn't available to all Mac users yet.
I expect the main constraint is the bandwidth that would be required for supporting 8 GB downloads to a large proportion of Mac users. Apple is happy enough with most Mac users downloading software updates in the order of 500 MB to 1 GB, and iWork comes in smaller than that.
It will happen eventually, but the infrastructure needs to be there first.
Bandwidth would certainly do it for Logic Studio and Final Cut Studio. Both weigh in at dozens of GBs after everything is unpacked. And much of that is stock media files which can't be compressed a whole lot.