No account belonging to MtGox has been seized or frozen. The accounts in question belong to Mutum Sigillum LLC, an US payment provider which MtGox uses. We do not know why Mutum Sigillum LLC were targeted. MtGox is affected, but I doubt they would ever get invited to court to defend another company.
They could intervene if they're affected in some way, such as having a claim to some or all of the money in the account. I have no idea what Mutum Sigilum LLC is other than a Delaware corporation that has been associated with Gox and apparently processes payments for it.