I think you're correct to point out that "in Telstra's case the government did effectively grant them a monopoly" in the first place.
The specifics I'm talking about in the Comcast case is that government is continually granting the monopoly privilege by preventing other companies from creating infrastructure (laying wire), even if a company wishes to do so.
This is precisely what allows Comcast to charge whatever it wants, begging more regulation down the road to force Comcast to charge 'fair' prices ('fair' in the minds of the legislators).
If any competitor was allowed to enter what is an enormous market it would provide continual downward pressure on consumer prices. The monopolistic privilege granted to Comcast leads to higher prices, as consumers can only pay the high price or forego service.