During the time that Freddie Gray was in the paddy wagon and they were accused of driving like maniacs, no one’s life was in danger other than Gray’s. The issue isn’t the arrest as much as the possibility of them carrying out an illegal “punishment”, resulting in Gray’s death.
They have a history of doing that. Not only to career crooks like Gray (who was more of a small-time dealer and thief than a violent hardened crim), but also to university librarians who filmed the cops responding to a noise complaint and may have displayed some attitude.