The whole _Keyword thing is to make it easier to port from older C standards to the new standard. It's not really supposed to be programmer-visible, only a workaround for compilers. There is an extensive explanation about the reasoning behind this solution in the spec. Read that and come back if you still have a problem with making forward-compatibility hacks.
The Cxx committees try really had to make everything backwards compatible. This includes trying to not step on existing namespaces. These names, along with mtx, etc., are just going overboard though.