Many criminals choose to use bitcoin for a billion reasons. I don't blame them. In fiat money, if police finds out that a drug dealer sells drugs, they arrest him and then his millions go to government.
If he had (let's say) 1000BTC what would police do? There is no law that says "Police can then take his private keys and move his coins to a government's address". If the police cannot do something with the money then they will leave them to his address. Will these coins get lost? I don't think so. When criminal gets fired from the jail (if he went) he will still have his fortune, right?
In 2013, silk road's owner had saved in bitcoin 400,000$ (by that time's price!!!).
When police investigate a case, and put one under arrest. After that, police can investigate further and if they can get private key, they can seize bitcoin from that one. It is not always the case will be resolved like that. There are two endings and depends on different cases but as the truth is, police often collects many spy information before arresting someone because of suspicious or fraudulent activities.
It means police mostly can seize illegal bitcoin, not all 100% but significant amount.