it shows banner ads for companies including SouthWest
Airlines on pages which include them in web search results.
It's not just 'If you search for Southwest Airlines'. It's if in the list of results for the thing you searched for there's a Southwest Airlines page. It could be coincidental but suddenly you have a large advertisement at the top of your page.
It wouldn't be outside the bounds of possibility for this to encourage Google to rank Southwest Airlines pages more highly so that they can show the banner, thereby generating more revenue.
I only see the ad when I search directly for "Southwest Airlines". For example, search for "Southwest check in". Southwest Airlines is the number one result, but there's no ad on the page: https://www.google.com/#q=southwest+check+in
It wouldn't be outside the bounds of possibility for this to encourage Google to rank Southwest Airlines pages more highly so that they can show the banner, thereby generating more revenue.