Sure, but again you're issue isn't with the police it's with the city (specifically clerks office) approving this, and they likely approved it because on legal grounds they didn't have any real way to deny it without getting sued and settling, handing Nazi's a bunch of money.
In Detroit, (unlike Boston for example), the only approval you need is from the city and not from multiple other departments. If you're approved and you pay your fee, it's a done deal.
The police knew what their intentions were (as with anyone who can put a pulse on the situation and make basic logical assumptions), but that doesn't inherently mean they can legally detain or prevent them from their planned and approved protest.
It's why their attorney was there with them and they taped the entire little hate walk. Best case scenario for them is they have someone throw a brick at a Nazi's face and a violent riot breaks out where fascists get to gun down "the assaulters" in self defense.
Second best scenario was they get to sue and make a boat load of money for their organization for any denials of their rights.
