Why does stock Mac OS X allow me to rip CDs but not DVDs?

This could get very technical but the answer is actually relatively straightforward.

The act of "ripping" takes place in 2 stages, reading the source material, and writing out a converted version of same.

For music this is straightforward, mainly because the technology is old enough to predate stuff that was later incorporated into DVDs. Music CDs do not have any form of encryption. The data is laid bare for you to use as you see fit. This means anyone can read it without requiring the use of a licensed decoder which you can them laden with prohibitions on use. Once you can read it, you can convert it, so long as you have a suitable encoder, which Apple provides to allow encoding into the formats you list.

For a movie, this is different. The source material is encrypted and copy protected in a way that Music CDs never were, and never will be. This means you need a licensed decoder just to be able to read them. The terms of the license will often dictate that you can only decode for playback, and not for conversion. Most often this is purely a legal distinction, and not a technical one. Obviously Apple have to abide by the terms of these licenses or they could not provide you with them, and as such they will not allow you to subsequently decode a film, then instead of watching it immediately encode (rip) it into a different format. They do provide an encoding mechanism for movies, but it will only work on unencrypted and decoded files. Of course, there is nothing to stop other programs that Apple do not supply from ignoring such paper restrictions (or providing replacement decoders that are less inhibited in what they allow), and happily ripping away.