You're calling this addition because it's a form of "adding" discounts to sale prices.
A lot of outlet stores have advertised discounts of, say, 20% off everything in the store plus 50% off certain items. This results in a total discount of $1 - (1 - 20\%) (1 - 50\%) = 60\%$, rather than the 70% you'd get by adding things together straight.
If it's 30% off everything and 60% off a certain item, the total discount would 72% instead of 90%.
And 40% off plus 30% off would be 58% off, as some people have mentioned.
In fact, the real numbers in the interval $(0,1)$ form an abelian semigroup (or a monoid if you include 0) under the operation $a + b - ab$, just like addition over the positive (/nonnegative for the monoid) real numbers. So it can rightfully be called "addition" in that way too.