The cops quite obviously don’t think owning a Pixel makes somebody a drug dealer. But if they arrest or detain a suspect then owning a Pixel flashed with GrapheneOS isnt exactly a sign of innocence. Even if nothing could be extracted from the phone, I’m sure a judge and jury could be convinced what they were doing if they have such a device in their possession.

Also, regardless of the security the OS claims to have, most criminals are not the brightest and I bet some can be squeezed to hand over the key or the phone can be unlocked with a face id or fingerprint. It also motivates the cops to do what they’ve done in the past where they have compromised supposedly secure operating systems or apps and installed backdoors.