Writing on bombs isn't something rare, British, US, French heck airmen from every other air force in the world do exactly the same. Believe it or not quite often even civilians are allowed or even invited to do so as a perk or a honoring service.

Here is an examples for Jordan http://i.dailymail.co.uk/i/pix/2015/02/05/255F9C3200000578-2...

https://www.google.co.uk/search?q=jordanians+write+on+bombs+...