"Allowed" is a funny way of phrasing this. The technology Microsoft has built to run x86 Windows apps on ARM didn't exist when Windows RT was released.
I think that "allowed" is correct. A developer couldn't just recompile their desktop-style app into an ARM binary for RT, for example. They'd refuse to run, and not because of the CPU architecture. The only way to get third-party apps was through the Windows store, which only supports Metro apps. Metro apps had a limited API available to them.
