Tether has been reluctant in many cases to freeze addresses reported to it. At least historically it would not do so if the coins didn’t belong to its direct customers (mostly the large exchanges), rather than individuals who got them from their customers and did crime.

https://archive.ph/6nvkc

The other major issue is it’s easy to get the stablecoins, move them around, cash them out, and by the time Tether freezes them the criminals have already been paid (in dollars, which is what they want really).