In the USB spec a "power thief" can only draw up to 500ma (0.5 A), and that is exactly what as Raspberry Pi is as it only has the two power leads connected to anything on the power in port. Some smart chargers will put a current limit on such devices, which is fine according to the spec, but not great for the real world.
That said if the resistance across the poly fuse is anything other than a few ohms, it is a rabbit of negative euphoria* and will need replacing.
* For those who haven't found "Cabin Pressure" that was on Radio 4 your seriously missing out...