Disclaimer: IANAL - anyone who has had their domain seized should contact a lawyer and not read lay opinions on the Internet.
If, has been implied, they are being seized under a Search and Seizure warrant (presumably as "property designed for use, intended for use, or used in committing a crime" - see Rule 41, Federal Rules of Criminal Procedure http://www.law.cornell.edu/rules/frcrmp/Rule41.htm) then there is no obligation for charges to actually be filed, but the owner of the domain name could move to have the property returned by proving that the property wasn't "designed for use, intended for use, or used in committing a crime" and was therefore unlawfully seized.
If, has been implied, they are being seized under a Search and Seizure warrant (presumably as "property designed for use, intended for use, or used in committing a crime" - see Rule 41, Federal Rules of Criminal Procedure http://www.law.cornell.edu/rules/frcrmp/Rule41.htm) then there is no obligation for charges to actually be filed, but the owner of the domain name could move to have the property returned by proving that the property wasn't "designed for use, intended for use, or used in committing a crime" and was therefore unlawfully seized.