Why Singapore Airlines when Singapore has only one international airline?

It's what the company chose to call itself. An "air line" is a line through the air between two points. So an international carrier like Singapore Airlines flies many different routes using many different "air lines", combining the two words into one. There's a US company called "Delta Air Lines", which uses the two words.

The term "line" to denote a route comes from ocean shipping, which is why big passenger ships are call "ocean liners" and shipping companies have names like "Holland America Line"and "Cunard Line".

It's a proper noun.

It can be called anything it wants, even Cool Singapore Airlines

Why do they call it "Fox News"???