The problem is that you don't understand Scala operators. There are none. That + sign there is simply the name of a method in the class of the object preceding it.

So you invoked print("5".+(5))

What else would you expect. The nice thing about Scala is that you can leave out some of the extra syntax that is not needed for the compiler to decode the meaning.

Now as for why the String class has a + method to do string concatenation? I don't know but this is far from the first language that I have come across that conflates string concatentation with numeric addition. Maybe it comes from the Java libraries? In any case, it is not really a problem to most people because we follow the maxim, if it hurts, DON'T DO THAT!