I know this isn't a big deal for me but why do guests get to post answers but not post questions?
This restriction only applies to Stack Overflow (and a few other Stack Exchange sites, see below); you don't have to register to ask questions on most other sites in the network.
The change was made because Stack Overflow has grown in popularity and the number of hit-n-run questions by unregistered users was getting out of hand. It's a quality control measure.
See Encouraging users to create an account (and keep it) as well, specifically Jeff's follow-up comment:
@eds it still works, just not at the scale of Stack Overflow. And to be brutally honest, the important part is unregistered answers; we don't really care about putting barriers in front of question askers. See http://blog.stackoverflow.com/2011/06/optimizing-for-pearls-not-sand
The complete list of sites requiring a login to ask a question is:
- Ask Ubuntu
- Electrical Engineering
- Meta Stack Exchange*
- Movies & TV
- Software Engineering
- Stack Apps*
- Stack Overflow
- The Workplace*
- All per-site metas (including those that allow <5 reputation users to participate)*
* Sites with this symbol also require registration to answer questions.