The server wasn't in the USA and from all the information I've read they claim the relevant foreign interests cooperated with them in obtaining the server image.
And I'm pretty sure they would have had NO trouble getting a warrant in the USA if they had a credible lead on the SR server. And any credible hosting company would GLADLY cooperate to the fullest extent with such a warrant. Generally hosting companies do not care what you're doing on your server and won't get in the middle of disputes UNLESS:
A.) You're doing something illegal
B.) Your actions pose an untenable threat to the network or other customers
>from all the information I've read they claim the relevant foreign interests cooperated with them //
The law they claim to have used requires that the requesting country act as if the computer being searched is in their own company, all protections that would apply domestically must be applied in order to request the search lawfully.
If they needed a warrant to get the local search done (eg in Pennsylvannia) then they needed one in order to use the 2001 Convention on Cybercrime to request that Sweden perform a search (and imaging) for them.
This appears to be a hole in the FBI argument - as you say, they're claiming they didn't need a warrant or to provide protections that would normally be applied if the server were in the USA. But the Convention used - by my reading at least (see my other post for links) - contradicts that specifically demanding that the normal domestic protections and procedures be applied.
And I'm pretty sure they would have had NO trouble getting a warrant in the USA if they had a credible lead on the SR server. And any credible hosting company would GLADLY cooperate to the fullest extent with such a warrant. Generally hosting companies do not care what you're doing on your server and won't get in the middle of disputes UNLESS:
A.) You're doing something illegal
B.) Your actions pose an untenable threat to the network or other customers