This is more of a problem of how things look like they are funded.
Customers still pay the same price, but instead of $60 for Internet service, it may be $50 for Internet and $10 somewhere else. Or of course the city can tax the heck out of one subgroup of people and redistribute the funds.
I'd actually be OK-ish with a law saying that municipal broadband has to be self funded after initial rollout, I imagine that would maintain sufficient competition.