Casting away immutability

[Sergei Degtiarev via Digitalmars-d-learn]

I can't understand how cast coexist with immutability. Consider 
the code:
	immutable immutable(int)[4] buf;
	auto x=buf[0];
	auto p=buf.ptr;
	
	auto i=cast(int[]) buf;
	i[]=1;
	
	assert(buf.ptr == p);
	assert(buf[0] != x);

I've just modified immutable data, is it normal?
Another, ever simpler, code:
	immutable a=0;
	auto b=cast(int*) &a;
	*b=1;
	assert(&a == b);
	assert(a != *b);
Last two assertions both succeed and really puzzle me. What is 
pointer in D at all?