A: Performance wise, a tailwind will have the same effect as using a shorter runway or having closer obstacles. There is nothing stopping a pilot taking off with a 10 knot (or 15 knot) tailwind provided that he has accounted for it in his takeoff calculations. There will be a weight decrement.
B: Have a look at FAR25, you will probably find that all aircraft must be certified to a minimum of 10 knots tailwind. Manufacturers do not want to spent more money than required to certify an aircraft, they will therefore only flight test to the required limits. Our newer aircraft have a 15 knot tailwind limit, it was offered by Boeing and MDD as a cost item.
Mutt