Many uses of strchr do write via a pointer derived from a non-const declaration. When we introduced const qualifier it was noted that they were actually declaring read-only access, not unchangeability. The alternative was tried experimentally and the consequent "const poisoning" got in the way.
I believe C is doing the right thing. Const as immutability is a kludge to force the language to operate at the level of data structure/API design, something that it cannot do properly.
