The reason they have to filter out artificial sources, and the only reason they're a problem, is we are literally right on top of the source. These aren't even as far away as, say, Mars. New Horizons, for example, can't communicate with Earth unless it points its transmitter / receiver directly at us. And that's in our own solar system! Only a few light hours away.

Just think about it. The signals are going to be so weak by the time the photons get here they will be indistinguishable from the CMB.