Why are we allowed to change values of “const” qualified variables?Why pointers are allowed for this,but not assignment?

Question

Consider the following 2 programs prog1 and prog2.Here if I try to change the value of the const qualified variable i

Answer 1

Why are we allowed to change the value of a read-only const qualified variable in any circumstance?

We aren't. I don't see why you assume that, nor which example shows that.

why the discrimination between changing the value of a read-only const qualified variable using a pointer (which is allowed,with a warning)

Again: it's not allowed, hence the warning. (Warnings are to be taken seriously - you don't seem to give any significance to them...) It's just that the compiler doesn't know that the pointer points to some const-qualified object (because it's declared as non-const T *).

As to why changing the variable works:

Explanation #1: it's undefined behavior (a constraint violation), so it can do anything.

Explanation #2: probably it's just stored on the stack like local automatic variables, and you can indeed change it.