There is "actual" water in the beer. Even if the alcohol in it (which is a very little amount in the beer) will cause more frequent urination, in a desert survival situation that water (urine) must be collected and re-used anyway.

For the calories, you are wrong, each beer can is more than 150 calories, that's about 2 large eggs each!

Also, I can easily google articles calling strong diuretic properties of beer just urban myth: http://www.socialanxietysupport.com/forum/f24/coffee-and-beer-cause-dehydration-a-myth-22964/
and
http://www.dshack.net/2011/11/no-coffee-doesnt-dehydrate-you-and-neither-does-beer.html

The last one shows a simple math of intake and diuretic release, so even if they don't collect urine (which is nonsense in the given situation) they could still benefit from the beer water intake.