If Jimmi T's information is sound, then the OPP commissioner is really just worried about the optics of the arrest. That's nothing BUT politics.
The good news is, the optics aren't hard to fix. And it's a great political move:
Assuming the detectives are trying to build a case for blackmail based on the wiretaps, then the obvious next step is to interview the victim to confirm that blackmail actually took place. So detectives invite Ford to talk about the blackmail. Seems to me there are two possible outcomes:
1) Ford details the blackmail and the blackmailers get charged, thus paving the way for his own arrest.
2) Ford refuses to talk to the police, so won't give the evidence needed to support blackmail charges, thus paving the way for his own arrest.
If it's 1) then the police appear even-handed and fair. If it's 2) then police have the perfect key message: "We actively pursued blackmail charges based on the wiretaps, and invited the alleged victim to speak to us. But Mayor Ford refused participate in the investigation. Without sufficient evidence, without a victim, and without a confession, we couldn't proceed."