They even cover the taxes if you're in the 25% marginal tax bracket:
One Hundred (100) Grand Prize Winners: One Million (1,000,000) MileagePlus award miles. Grand Prize will also include $7,500 cash (awarded in the form of a check to the winner) that may be used toward the payment of any/all applicable taxes associated with the prize. Total Approximate Retail Value ("ARV") of Grand Prize: $27,500 each.
For the IT security bug bounty they cover none of the taxes if you earn the 500k mile bounty.