No, technically, you're correct; CASS was SPECIFICALLY designed to allow COCKPIT jumpseating on other carriers. However, many of these carriers have simply written their policies to only allow CASS participants a jumpseat, whether it's in the back OR in the front of the bus.
Prior to 911, it was based on reciprocation; you have a seat available on your bird, so we'll offer you a seat with us. After 911, most of the reciprocating agreements went out the window. However, if you get put into the CASS system, you have, by definition, a seat available to trade, and that saves everybody the hassle of getting reciprocating jumpseat agreements between all the different airlines.