Why the c# compiler requires the break statement in switch construction?

The compiler doesn't so much 'need' the break statements, it demands them.

This was a design decision. It keeps the code semantically close to C and C++ while eliminating the pitfalls of fall-through that was always a debatable 'feature' of the C languages.

The break statement in c# was a design decision by the creators of the language...Essentially they wanted an "unambiguous" break statement, a break statement that would only work one way. In short, they didn't want fall-through, and if they had just prevented fall-through without including "break," it would have broken backwards compatibility with c++.

Fallthrough is allowed if the case expression is empty:

case Foo:  // fallthrough allowed.
case Bar:
    Console.WriteLine ("Foo or Bar");
    break; // required

That it is not allowed is a common misconception in the same league as "you can't assign values in if-conditions" ^*

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^* You can. The rule is just that only boolean values are allowed in if-conditions, and x=false with bool x; is a boolean value.