If you've done any university-level maths you should have seen the + sign used in many other contexts than adding numbers, why should that be a problem when programming?

There is usually another operator used for concatenation in math though: | or || or ⊕

The first two are already used for bitwise and logical or and the third isn't available in ASCII so I still think overloading + was a reasonable choice and doesn't cause any actual problems IME.