I was about to say, that it is simply a must. But I looked at the JLS, and it is not that strict. From the JLS's perspective, it is left to the compiler to choose whether to set such a restriction or not.
Practically spoken - common compilers do have that restriction, and, as other already explained, it's much easier for the compiler to find a compilation unit or for a classloader to find a class file with such a restriction in place.
