Well with the tag of privacy, pseudo-anonymity in the name of Bitcoin, it's rather easy to create an image of a medium mostly being used for criminal activities. Though honestly, with enough influence (or money), anything can be hidden from the public eye, and as long as they don't know, no one would really admonish them no? Ignorance is bliss perfectly suits the masses here tbh. Plus, it's honestly more likely for illegal activities to happen on centralized businesses, which just makes it more likely that something is happening under the table most of the time.
This isn't because Bitcoin is pure and inherently repels criminals from it, it's because it's generally not well adopted, even among criminals. Plus, privacy is far from perfect now, so moving large amounts of money and not getting detected later would be quite hard. But Bitcoin has privacy improvements in its roadmap, so if we imagine that Bitcoin would one day be as popular as banks, than all the criminals will switch to it too. And if Bitcoin will not get more popular, than there's really no point to be happy about banks helping criminals.
Even if we do consider Bitcoin being used instead of fiat, the money originally would come from centralized companies, either through moles or insiders, which honestly still makes the ones at fault the companies by being quite careless. There would still be a window or frame that criminals would use fiat to transfer funds to Bitcoin, so if securities could capitalize on that, and as long as they know that these criminals would go 100% for exchanges or p2p to trade BTC for their stolen fiat, then they can anticipate it.