The reason for that is, that i.e. U and Ü are pronounced very differently and often change a word entirely. So the added e makes sure that the reader knows, that there shall be umlaute.
In standard German, Koln and Munchen would be considered incorrect. Of course, that doesn't really matter on Jetpunkt.
"Istanbul, formerly Byzantium and Constantinople, is the largest city in Turkey and the country's economic, cultural and historic center. The city straddles the Bosphorus, and lies in both Europe and Asia, with a population of over 15 million residents."
Sorry, Istanbul. Only one I missed