Well, that's true, technically. But can't you name your functions things like "++"?

Yes. Because Scala doesn't make up random rules about how you name your methods.

If you name your method addAll or ++ is pretty much the same. The same rules apply to both. Both can be called the same way:

    coll1.addAll(coll2)        coll1.++(coll2)
    coll1 addAll coll2         coll1 ++ coll2

Those "random rules" make the code base much easier to read. This kind of thing is a big step backwards for people working on large projects, especially people who need to come up to speed on a large code base.