I don't know if the technique is forbidden because it was said that if Naruto was to read that scroll that he would somehow be able to release the Kyuubi blah, blah, blah and we know that wasn't true.

Also the first few episode/chapters in the series were absolute crap, I don't think Kishi knew back then exactly what direction he wanted the whole Naruto/Kyuubi angle to go in. If the scroll and what was in it is such a big deal I would think that the third would have made more of a fuss about it so I just chalk all that up to the series just starting off.

The shadow clone technique can't be forbidden because, forbidden techniques have a tremendous risk to the user and although using it would quickly use up the majority of your chakra unlike chidori it doesn't seem to have much risk to your life force. All one needs to know to be able to use the shadow clone tecnhnique is one seal, I think a forbidden technique would be more difficult to perform than that and secondly it was said that the technique is taught to all jounins so if the technique is forbidden they wouldn't still be teaching it.