I beg to differ. Most do have an Exit option. Especially the older ones. Even on my 808 many of built-in applications have Exit - Mail, Store, Music Player, etc.

Some apps have 3 ways to close

1. back arrow
2. Menu option
3. Hangup button

Some have only 1 or 2, and 3. And some like Camera can only be closed using 3.

Coming from some other company I would say it gives you choice, but coming from Nokia it's more of them being inconsistent and incompetent. One of the changes they made in the Belle upgrade was supposed to be an improved keyboard. The English input keyboard was a marginal improvement, but the Chinese input keyboard was a step back from the Anna version. And to make things worse some remnant of the Anna keyboards are still present after the upgrade and some applications would use those instead of the new ones.