They're being good Christians in a world that, in the end, hates Jews and Christians for some reason or another. Look at how the so-called caliphate of the 'Islamic State' butchers Christians in Iraq and Syria and see how some of the Arab Christians would obviously cling to Israel as an oasis of stability in a climate of madness that would drive H.P. Lovecraft insane.
Israel is a bastion of safety in an area of the world that has been governed by darkness ever since the islamic death-cult rode out of Arabia 14 centuries ago- and I am a Christian (Catholic) and we are obligated by our faith to be good and loyal citizens of whichever polity we are governed by ("Give to Caesar that which is Caesar's and give to Gd that which is Gd's..") . Arabs (and Moors, etc.) by and large
used to be Christians or pagans before the islamic wars of conquest and forced conversions in the MidEast, North Africa, etc. to that horrid religion- and there are still populations of Christians in the MidEast that go back to the time of Jesus (some of them being purged by ISIS!).
The IDF has Druze soldiers so why not Christian soldiers? As we say in America 'Don't look a gift horse in the mouth.'
Keep this in mind friends:
"O you who believe! do not take the Jews and the Christians for friends; they are friends of each other; and whoever amongst you takes them for a friend, then surely he is one of them; surely Allah does not guide the unjust people." (Koran 5:51)