They only explained why it would be illegal in the US. Gitorious is in Norway, so they want Sony to explain why it would be illegal under Norwegian law, which is the only thing that applies to them.
The way the Norwegian law sounds is that the claim gives Sony an automatic temporary injunction while the matter is clarified, which doesn't seem completely unreasonable.
The question then would be on who is it incumbent to prove whether an infringement took place or not, and whether there is a penalty for making false claims.
Just for those interested in the relevant law... Sony sent a DMCA takedown, which is purely US law. But the EU has implemented EUCD (EU Copyright Directive) which, for practical discussion, does much the same thing. I think that is what they referred to when taking down the content.
Ahem, oops. :( That was a silly basic error, sorry.
EDIT: my Norwegian is non-existant but as far as I can make out they have their own equivilant (to the EUCD and DMCA) implementation of the WIPO treaties.
Well, to be fair, you can submit a counter-notification letter that commands the ISP to put the content back online and that doesn't require any proof either.
Actually not quite. An ISP can ignore DMCA notices. This waives the DMCA safe harbors protection, and allows the alleged copyright owner to sue the ISP. Then the destiny of the content in question is then decided by the court, but the ISP becomes potentially liable for damages. An ISP cannot be liable for damages, if it complies with a DMCA notice, even if the content is actually infringing.
Sadly it doesn't. I don't blame Gitorious for following the law, and I support them fighting back.