Their answer to the first one is wrong. Square root of a positive number gives two results, plus and minus. I haven't checked my working, but I believe the correct answers are 15, -9, 21 and -3. Rather disappointed that I caught the trick question and the questioners, apparently, didn't.
Indeed, and I think the reason it's done is simply to make square-root an easy-to-deal-with single-valued function rather than a multifunction. Remember the quadratic formula: the ± in it is in addition to the square root, not part of it.
On every notable standardized math test in the US, and on any commonly used calculator, a square root is non-negative. Another example: http://www.wolframalpha.com/input/?i=sqrt(x)