What is the scope of a 'while' and 'for' loop?

Anything declared in the loop is scoped to that loop and cannot be accessed outside the curly braces. In fact, you don't even need a loop to create a new scope. You can do something like:

{
   int x = 1;
}

//x cannot be accessed here.

In the following examples all the variables are destroyed and recreated for each iteration of the loop except i, which persists between loop iterations and is available to the conditional and final expressions in the for loop. None of the variables are available outside the loops. Destruction of the variables inside the for loop body occurs before i is incremented.

while(int a = foo()) {
    int b = a+1;
}

for(int i=0;
    i<10;     // conditional expression has access to i
    ++i)      // final expression has access to i
{
    int j = 2*i;
}

As for why; loops actually take a single statement for their body, it just happens that there's a statement called a compound statement created by curly braces. The scope of variables created in any compound statement is limited to the compound statement itself. So this really isn't a special rule for loops.

Loops and selection statements do have their own rules for the variables created as a part of the loop or selection statement itself. These are just designed according to whatever the designer felt was most useful.