Not necessarily incompatible positions. It's perfectly possible you might want 16gb today but 32/64/etc. later down the line. In fact, based on apple's ram prices, this is the advisable approach, providing you can get a machine which actually CAN have the ram upgraded.
The author of the article wants a 13" laptop, and the 16GB option is only for 15" Macbook Pros.
No, 13" MBPs can get 16GB too.
Ah, you're right. They don't show the option until you've added it to your cart though, unlike the 15" model.