Even then, though, I'd still be very surprised if Google would respond to an offer of seat availability data from Southwest with "sorry, we won't let people search it unless you pay us".

Google works with a few airlines that don't pay them anything. However, Southwest (probably rightly) thinks that if people get used to finding Southwest fares on Google, they'll visit Southwest.com less frequently, increasing Southwest's reliance on Google in the long term. I'm nearly certain Southwest has sent a specific cease and desist to Google to NOT include its fares in results.

Yes, Southwest asking Google not to include their fares in results seems likely to me.

(This thread started with me arguing that "it costs money to be there" was not the reason Southwest flight prices aren't in Google.)

Isn't Google sending customers directly to airline websites to book?

So the way this is working is: Google, Hipmunk, etc. use GDS for queries, and then send customer to airline to book? Does GDS have any ability to sell tickets?

Has Google tried to scrape data directly from Southwest website and Southwest won't let them? Or is it just that Southwest will not provide APIs to make it easy for Google to pull the data?