Comment by dmbaggett

They were once required to do so (and may still be; I’m not sure at this point), at least if they want to offer flights to or from the US:

https://en.wikipedia.org/wiki/ATPCO

However, interpreting this data properly is decidedly nontrivial (>1M LoC).

Pricing does not imply booking is OK, though. And even circa 1999, Southwest hassled us (ITA Software) about even showing their fares, without us offering any way to book any flights on any carrier.