The reason they have oil originally is because it has a much higher boiling point than water, and also has lubricant qualities. Don't forget that these aren't trials specific brakes! They're XC brakes. Riding down steep hills/cliffs/whatever and dragging the brakes will heat them up. - The last thing you'd want is for your fluid to boil over, and the brakes not work!
Magura internals are plastic, so they don't corrode. Meaning that they don't really need lubricating as much as disc brakes, etc do. The pistons do go 'sticky' from time to time, but as said, a good service every now and then does them a world of good!
Water is favoured amongst trials riders as; it's free, it has a much more responsive feel at the lever, it's free, it's not as messy to bleed with, it's free, it doesn't contaminate your rims/pads. Did I mention it's free?
Just chill out, and get the facts straight before you go waving your arms in the air going mental!