You're not interested in amplifying DC voltages since they heat up the coils of the headphones.
A capacitor has an impedance caracterized by Z=1/jw*C, where C is the capacitance and w is the angular frequency in rad/sec. For DC, i.e. w=0, impedance reaches infinity.
The impedance is not always so because differenct capacitors have different equivalent circuits, but it is a very good asumption for DC and up to a few MHz.
RSA probably use some different topology i'm sure. There are other ways to detect DC which does not involve coupling capacitors, which effectively can shut off the amplifier if the dc voltage input is too high.
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