There are no seized assets anymore.
The police made their copies and gave the servers back to the vendor, which was NOT MegaUpload. That vendor is due payment from MegaUpload, which MegaUpload are not able to pay because their assets have been FROZEN. The assets are frozen because of the money laundering charges, which enable RICO seizures. RICO* laws were passed in order to limit the ability of organized crime to move their money about and continue their criminal enterprise even though a given individual in the organization might have been arrested, tried, and convicted. Also it seems to have increased the longevity of star witnesses in the cases of such organizations.
On the issue of the data, the police are issued a warrant to cover certain items. Items not covered by the warrants can't be kept. Hence the "selective" copying: they selectively copied the data covered by the warrants. Copying more than what is covered by the warrant is illegal and has the high probability of getting the case dismissed. The copied data must be made available to the defense. If the defense wishes to search for more data, that's on their dime and at their risk for civil suit if THEY breach someone's rights.
*I actually come down on the side of people who think RICO needs to at least be re-written if not repealed because of abuses which have happened with non-organized crime defendants, but the law is what it is, and those are the reasons given by those who support RICO for keeping it as is.