This is all purest speculation, but I am assuming that Gringott's doesn't impound an account just because the Ministry wants them to:
Ministry Official: We'd like you to impound the account of a convicted criminal.
Gringott's Representative: We are glad to cooperate with any lawful instructions of the Ministry. What is the criminal's name?
<PERSON>: <PERSON>.
<PERSON>: Ah, yes, of the ancient and wealthy <PERSON> family, I presume?
<PERSON>: Yes, that's the one.
<PERSON>: Very well. Of what crimes has he been convicted?
<PERSON>: Thirteen counts of murder, and three counts of conspiracy to commit murder, and numerous lesser charges.
<PERSON>: That does sound like a grave matter. Although we regard the possibility as minute, it may be that as a customer of our establishment, his activities have compromised our security. May we review the records of the investigation, and the transcript of his trial?
<PERSON>: We'll forward the records of the investigation shortly.
<PERSON>: We look forward to receiving the records. When may we expect the trial transcripts?
<PERSON>: Err, there aren't any.
<PERSON>: The Wizengamot did not prepare a transcript of his trial?
<PERSON>: Um, there was no trial.
GR: There was no trial?
<PERSON>: That is correct.
<PERSON>: Unless I am mistaken about Wizarding law, that means that he was not, in fact, lawfully convicted of these crimes.
<PERSON>: But he is quite dangerous. 600
We have him in Azkaban at this very moment.
GR: Nobody applauds the confinement of dangerous criminals more than we here at Gringott's. However, our policy on the impounding of accounts is very specific: The person whose account is to be impounded must first have been lawfully convicted by the full Wizengamot.
<PERSON>: But he is a deranged murderer!
GR: That is beside the point. We cannot proceed with your request until we have received a full transcript of his trial. A simple accusation, no matter how unimpeachable the source, is not sufficient basis.
<PERSON>: But he has killed thirteen people!
<PERSON>: I am sorry, but until we have reviewed the trial transcripts, he has killed nobody. Now unless you have proper documentation for any other requests, I must ask that you allow <PERSON> here to escort you out. There are other customers waiting. 691