So what goes in to calculating the "actual" RAM limit? Just the fact that 4GB sticks are the highest capacity currently manufactured?
It's a completely arbitrary limit in the kernel. All Intel Macs have had PAE, so the hardware supports 64GB, but the kernel does not.
I would guess that, at the time the computer came out, that was as much RAM as they could cram in there to test the kernel against. Apple have never in the history of the company existing added support for an existing hardware feature after a product was released without support (except 802.11n in G2 MacBooks, but they charged for it), so don't expect them to "fix" it ever.
Yeah, they're just the only PC manufacturer that includes a DRM chip specifically so that you can run their OS.... 
Apple is
definitely not the only PC manufacturer that includes TPM chips in their computers. Most laptops have them, actually. Every machine Dell currently sells has one. I've even seen some motherboards on NewEgg that have them.
Console and cell phone manufacturers get away with locking down their products. I don't think it's insane for Apple to do the same. Also, it's actually just a TPM chip, not specifically designed to perform DRM; it's used for legitimate purposes too, such as securing the keychain. The chip is just some WOM (a private key store) and a simple processor optimized for doing cryptographic operations, so there's nothing inherently unjust about having one.
