Consider a transceiver in two states: dominant and recessive.
In the recessive state, the transceiver is open-circuit, and CM voltage is defined as the difference between instantaneous bus CM voltage and local ground. The receiver is thus susceptible: exceed its CM range and gibberish is received.
When a CMC is used, its voltage drop is small: the impedance at radio frequencies might be a couple kohms, but the transceiver is high impedance as well. Thus the CMC does almost nothing on receive. (At high enough frequencies, transceiver capacitance takes over and some filtering value can be had.)
In the dominant state, the bus voltage is either forced high, or the transmitter acts in parallel with another transmitter, and both share bus current to some extent. Either way, the impedance is low, or at least the voltage difference between bus and transmitter will stay low. The bus CM voltage is thus forced to local ground (or, well, somewhat halfway between GND and VCC, but referenced to GND is the point).
When there exists a galvanic (say DC or mains-frequency) voltage difference between two nodes, their alternating transmissions will pull the bus CM voltage up and down, thus emitting CM radiation coincident with the transitions into/out of dominant state. (This might be contained in a shielded structure, cabling, cableways, etc., or it may couple to nearby cables, or it may be exposed to space and radiate per se; I use "emitted" generally here.)
When the bus is long, its capacitance to surroundings, or more generally its CM transmission line impedance (whatever that may be; in general it varies with position along the bus, due to variable proximity to surroundings), is relatively low, and a CMC has some impedance to work against. Thus transmitter CM emissions can be reduced.
For the special case when a node is near/at the end of the bus, it can be terminated nearby, and that termination resistor can be tapped to obtain a common mode reference to the bus, independent of dominant/recessive transmitter state.
When a termination is tap-grounded (via bypass capacitor) and combined with a CMC, very high CM attenuation is possible (i.e., kohms into 30 ohms), and over a broad frequency range. Conversely, emissions are reduced, as the node presents a fixed (DC) CM voltage, rather than a dependent one.
Since there are at most two terminators on a bus, this isn't a general solution, but in the special case where nodes cluster only near the ends, or point-to-point communication is used, very high signal quality can be obtained even through a rather mean environment. It's basically like Ethernet without the transformers, but with enough CM input range that it can handle most anything else (and is also slow enough, and robust to errors, that the remaining stuff -- EFT and ESD most likely -- can be dealt with; something Ethernet doesn't have a luxury of, hence the transformers -- and CMCs).
Tim