Hi Brad,
Regarding Amateur (Ham) Radio, at least, there are regulations which stipulate which party must correct interference; these regs do follow common sense. In truth, most operators operate in a good faith manner.

I do agree with using the internet whenever possible; again, in Amateur Radio, this technology shift has been felt; many operators use packet-based communications, etc. via the internet.
Regarding interference "in the ether", we perceive the interference at the receiver, because that's how we hear it. There are occasions, however, when 2 distinct signals operating at the same frequency, but originating from different sources, can find themselves in the same space at the same time. It is for this reason that some transmitters (ususally FM) transmit at much higher power levels during the day than they do in the evening, because of ionospheric ceiling changes. I recall travelling back home from the NJ shore to my home in Northern NJ and there was a distance during which the FM signal 92.7 recieved by my car's FM radio kept switching between the South-Jersey based WOBM and the Long Island based WLIR, and that happened despite FCC transmission power regs.

for whatever it's worth :-)

Earl