Nobody has to do anything, but there are more or less standard naming conventions - which are generally followed because they make things easier for the customers.

A video game is usually titled in the app listing with the name of the game, not the manufacturer (I'm sure there are exceptions to the rule.) This makes it easier to find.

A publication - newspaper, magazine, etc. is usually titled with the name of the publication - not the name of the publishing company, because that's how people expect to find it.

Same with airlines - Delta is free to call its app "aardvark" if it chooses to do so; it would just make it very hard to find. However, if you look at the Android Market, KLM, Air France, United, American, Southwest, Alaska, Asian, Singapore, British - all title their apps with the name of the airline first in the name, so the app will appear alphabetically under the name of the airline.

The only airline I found that did not follow this common sense convention is Delta.

While Delta is free to do this, it clearly is an outlier.