If he had decided to supress the postcard for people with less than 1 USD balance, many of them would complain anyway, or worse. 

But be sure to check the rules with them.  According to some posts here, if they follow standard bankruptcy practice, they will define your claim as all that you deposited, minus all that you withdrew -- rather than using the USD and BTC balances in your account at some arbitrary date.   

But perhaps Mr. Kobayashi himself doesn't know yet how he will compute the claims.  This bankruptcy is probably virgin territory for the courts.