Up until now for all of those seizures that I've checked the registrar has always been located in the US. In such a case I understand that the domain name falls under US laws.
However, in this case it is a little bit unclear: The "ub.com" domain itself has been registered with an UK based registrar, but the nameservers point to a US company (UltraDNS). Still, from the evidence available it seems that the US directly tampered with the data at the registry (VeriSign):
* According to the UK registrar's WHOIS information the responsible nameserver for the domain is at UltraDNS.
* UltraDNS seems to return the 'correct' results.
* When asking the .com nameservers for the ub.com nameservers, they return two nameservers at cirfu.net.
* cirfu.net leads to the web page with the seizure information.
So yes, it seems that the FBI can indeed size any .com domain, even though it has been registered with a registrar outside of the US. If I'd be a company not based in the US now would be the time where I'd think about migrating to another TLD - I guess for online gamling sites even .ly domains would be a better bet than .com.