What is coming out of comments of persons much more law-trained than me is bringing me to some conclusions.
1. The actual law regarding copyright is easily appliable if we are talking about books. A book physically existing and a copy are two different things. Two files are exactly the same bits. A file can be copied in thousand of instances all equivalent, while a book will be different from a xerox. Problems of unlegal distribution of the copy are clearly found out when you can distinghish in between original and copy.
2. The same "physicity" implied in the aforementioned laws can be applied when I buy one thing in a country when it is legal and I bring it in a country where it is forbidden. There is a borderline in between the two lands which must be crossed, I have to declare the goods at custom. This does not apply to internet. If we were talking about digital copies which would remain digital copies, there could be a jurisdiction just for the web. The problem is that the digital copy can be transformed into a paper one, thus assimilable to a xerox of the book. If the book can be xeroxed legally in the land, there also the printed copy can exist. Otherwise the printed copy is unlegal.
In any case, to create this distinction in between the two things (xerox and printed copy) should be enough to add a header on all the pages (like what happens through sheetmusicarchive.net), which, IMHO, should also specify that the use of that copy is unlegal in <list of countries> until <year>. This list should be supplied by the publisher.
I volounteer to help in doing that work.
If we could then find a kind of agreement in between imslp and the publishers, something with a philosophy near to the creative commons or this kind of licenses, the statement regarding this licence should be printed out also.
Lastly, I have another doubt: have not understood one more detail: if it is OWNERSHIP of a copy of a unlegal work, the person to be prosecuted is the owner of the copy if he belongs to a state which prohibits that. If it were so, the actual situation would be perfectly legal as it is. If, instead, the free distribution of a copy is comparable to sell it, imslp would be guilty as well, unless if I assume that when I am on the imslp homepage I am considered as I were in Canada, so I am legal until I do not download a copy of the file on my computer. But I do not know if this is feasible, because when I am browsing a site the files are temp files on MY computer, so I have already imported them in a country where they could not be legal, thus coming back to the vexata quaestio regarding ISP filtering.
I had never understood like now that the world needs an internationally agreed law apparate for the web. Local laws cannot be applied in any possible satisfactory way.