Code wise, in order to open a @Window, you have to issue a /window command. So there's really no reason to have it trigger the ON OPEN event as well, as you already have code-control over when they open.
Right. If you already have to type the command to create the window, what's the point in not having the code (or alias) right there with the /window command? Putting it into an on OPEN doesn't really do anything for you.