It is against the National Electric Code to run power and
signal in the same raceway without a physical barrier between the two.
RE: external power 'bricks'...
One reason for the external power, I have been told, is that otherwise
if the power supply is within the modem, each modem design must be UL,
CSA, and whoever else approved. This takes time and money. On the other
hand, if the 110v (or whatever) is kept in the external transformer,
and only low voltage is allowed into the modem, all this time and
expense is avoided. The manufacturer can purchase common transformers
that are already approved for all their modems.