Maybe it's not illegal, but it definitely says something. The reporting is that Stone's attorney was set to let Stone turn himself in. He wasn't a flight risk. There was no need to make a 6am SWAT team capture of a guy that was set to turn himself in, while having it taped by CNN news for immediate broadcast.
This tells us that it wasn't about the arrest. It was about intimidating Stone (adding pressure to the process indictments) to get Stone to do something that he's so far refused to do. So, it tells us that Mueller has very little/nothing on Stone as it pertains to his overall case, and most likely even less on Trump.
This isn't me talking. It's former federal law enforcement expertise commenting on it.