Fraud-risk perhaps, but DoS potential is very limited given that southwest.com is behind Akamai CDN (the entity that is generating the above error message). Akamai distributes southwest.com content to hundreds of servers around the globe. If one server comes under attack, they can just shift traffic from that server to another one that is not under attack.
True, unless the target is Southwest. Some of the larger DoS's perpetrated by state actors are capable of and have overwhelmed CDN's (I don't recall if Akamai was previously or not). Null routing or deflecting IP blocks that have little business implications due to service area is not a bad practice IMO.