In C#, why can't I modify the member of a value type instance in a foreach loop?

Я знаю, что типы значений должны быть неизменными, но это всего лишь предложение, а не правило, верно? So why can't I do something like this:

struct MyStruct
{
    public string Name { get; set; }
}

 public class Program
{
    static void Main(string[] args)
    {
        MyStruct[] array = new MyStruct[] { new MyStruct { Name = "1" }, new MyStruct { Name = "2" } };
        foreach (var item in array)
        {
            item.Name = "3";
        }
        //for (int i = 0; i < array.Length; i++)
        //{
        //    array[i].Name = "3";
        //}

        Console.ReadLine();
    }
}

The foreach loop in the code doesn't compile while the commented for loop works fine. The error message:

Cannot modify members of 'item' because it is a 'foreach iteration variable'

Why is that?