How does this happen? Why do all these webcams have similar HTTP access URLs?
They use the same or similar IP Camera providers? Maybe that's just the 'standard' that IP cameras use as the URL for accessing the live feed.
Similar to routers - how is it the case that you still see many unsecured wireless networks? (I know the majority of ISP's now implement at least WEP as standard on their devices) Simply because that's how the users receive the devices and don't bother to change it; either because they're not bothered or aren't aware of the risks.