Haliburton does not "make" as much as you think. When they do contracts for the government, like in Iraq, Haliburton works from a price list the government will pay, period. On somethings Haliburton loses money, on others they make money. When it is all said and done, Haliburton makes about 1% profit. Of course 1% of a really huge, gigantic, enourmous, large number is a large number.
I live in the Houston area, where Haliburton's headquarters was located - they are moving - and have good friends who used to work for Haliburton. After tours in Iraq on Haliburton projects, they decided to work for the oil industry here in Houston so they would not get shipped out. They are the source of this information.
I am niether for, nor opposed to, Haliburton, just providing what is a probably oversimplified explanation according to former Haliburton employees.
My guess is Haliburton does the work because they are big enough to have the assets to get the jobs done - there are not many companies that are capable of doing these jobs, can keep skilled employees on the payroll, break even by just a little - and this is the big thing - generate goodwill with various governments around the world which allows Haliburton to make real money on other contracts - sort of a cost of doing business thing. This last paragraph is just my cynical view of these sorts of things in the world.