That might be the logical position but it isn't the legal position when it comes to criminal proceeds in fraud and other cases.
Imagine you saved $100,000 from working and spending less than you earned, all completely legitimiately and legally. Now imagine that you make another $100,000 from, say, importing and selling a few pounds of cocaine.
You might think the government may come along and seize $100,000 from you. No, they'll seize everything. It's really difficult to separate money and the government won't even try. All of that could be forfeit.
Let me give you another example: imagine you're in charge of purchasing for a company. You send out and pay invoices on that company's behalf. Imagine you skim off the top and add $1,000 in charges or increased prices or fake items to a $20,000 invoice. You send out a $21,000 invoice, keep $1,000 and then pay the company $20,000. This is fraud. Imagine you get caught. You won't be on the hook for $1,000. You may be on the hook for the full $21,000. The entire invoice is fraud at that point.
Now this may seem unfair or unreasonable. I won't argue that. I'll simply say, that's not how it works. So once SBF committed fraud, which seems to be the case, he basically has the reverse Midas touch. Everything he touches from that point becomes tainted and the government and the courts are within their rights to seize it.
If his parents accepted any proceeds of that fraud, even unknowingly, the government can start seizing their assets too. In the Nelson case I talked about originally, the government seized the bank accounts from Nelson's lawyers. Think about that.
As others have commented correctly and more pithily than I have: the issue isn't is civil asset forfeiture justified (it isn't) but why those same rules are being applied inconsistently, much to SBF's benefit as he's walking free today.