Not commonly known, but they do get a tax break, of sorts. Re-enlist while serving in a combat zone, and your bonus is tax-free. If you spend the majority of the month in the combat zone, your base pay and entitlements are also tax-free. I know of a young E-5 couple, each in the 3C0X1 AFSC who re-upped for 6 years each, and each had a "3" Selective Re-enlistment Bonus, or SRB. That's 3x your base pay for the number of years you re-enlist for. Each of them got $43K for their re-enlistment bonus, and since they were savvy enough to re-up in the combat zone, all $43K was theirs, vs. giving a quarter or more of it back to Uncle Sam. That's $86,000 between the two of them, and they were in their mid 20s, so they still had their Zone B and Zone C bonuses to look forwards to, if they planned on making a career of it. (And assuming the bonus rates stay the same) The law prohibits single bonuses exceeding $60K.
A lot of my pay was tax free, depending on how much of a given month was spent in a designated combat zone. It's called the Combat-Pay Exclusion, and I'd wager the Army, Navy, Marine Corps, and Coast Guard have similar exclusions.