They're freezing up to $45M to pay back the amount allegedly defrauded from Medicare: http://www.csmonitor.com/USA/Justice/2015/0514/If-a-defendan...
It would reward people for spending stolen money first if only the stolen money itself could be frozen, rather than simply allowing them to freeze the amount stolen, whatever accounts they might have to freeze to do so. That's the simple, logical consequence of separating the accused person's funds in the way that's being requested by petitioner. You spend stolen money and your regular assets can't be frozen, but if you spend your own money, the stolen money can't be. That simply rewards one for spending the stolen money and not their existing assets by privileging the others from being frozen.
Thus many of the justices, from Kagan to Alito, question how petitioner's request makes any sense.
Finally, although I realize you never said it was, due to the Breaking Bad reference, I feel compelled to continue to point out that this case does not involve drugs. Instead, it involves allegations that "bribes and kickbacks were paid to prospective patients who agreed to sign up for home health care they did not need or never received" in a scheme to defraud Medicare.
The law has a substitution rule that lets them freeze sufficient funds to recover the amount taken by fraud. Prior precedent allows this. Petitioner claims a violation of their Sixth Amendment rights.
It's hard for me to see why someone with a very expensive attorney can even make this case. Just how expensive an attorney does one need before their Sixth Amendment right is satisfied?