Let's take this const map:

  const std::map<int, int> constMap = { {1,1}, {2,2 } };

Or take a const map copy-constructed from from a non-const map, if you wish.

How is this not immutable data, and "only provides a read-only view"?

It is linguistically guaranteed to be non-mutating - const-casts excepted of course.

All languages that are constructing const data types on the fly are putting them in a writeable pages in memory, after all.