They do have different batteries but you can remove the new one by unscrewing three screws and disconnecting a cable [+]. The battery has even a pull tab which allows you to easily lift it out of the case.

That’s a little more involved than previously when you only had to open a latch and pull out the battery but it’s pretty much exactly as hard as replacing the hard drive.

[+] Here is iFixit’s guide: http://www.ifixit.com/Guide/Repair/Installing-MacBook-Pro-15...

It was my understanding that parts that are officially consumer replaceable parts need to be more structurally sound for safety compliance (e.g. in case it is dropped when it is outside the machine), and that this extra battery casing takes up significant space—space that could otherwise be used for more battery.

Aha, here we go:

"With the MacBook Pro, Apple engineers went further. To make the lithium-polymer battery even bigger, they looked for extra space within the enclosure. And they found it in an unlikely place — the removable battery itself. Removable batteries take up a lot of space. They require infrastructure that takes up space. The batteries themselves have to be encased in bulky plastic housings to withstand the wear and tear of handling. Those housings take up more space. And the notebook enclosure has to include mechanisms for a separate battery compartment, such as latches and a door. All those parts take up space that could be used for the battery itself."

Source: https://www.apple.com/macbookpro/battery/

Apple's marketing copy gives a plausible reason for their design decision that makes them look good? Shocking. Apple's site is not going to say, "if the battery was removable, we couldn't charge $99 to replace it," which is probably the real reason.