bypass capacitors are used to filter out (typically high frequency) ripples from the power supply or to the power supply, and to control stray inductance.
decoupling capacitors are used to block dc from ac signals, and sometimes people use decoupling to mean bypass (as in "power supply decoupling").
Because that's what they are - you're thinking of COUPLING capacitors, they block DC and pass AC.
Decoupling and bypass capacitors are mainly to connect two points to the same AC potential, all supply rails should be the same as far as AC is concerned.