They can ride because of "familiarization" flights and whatnot. That is a very optional part of their job description though, and were it not for the involvement of pilot union(s) (mostly ALPA but others helped) there wouldn't be a CASS system in the first place for them to pigy back onto.

Its rediculous to take pilots from carriers that don't reciprocate anyway though, with a few limited exeptions like a period of time when a carrier suddenly shuts down or is on strike, etc.

At some carriers, non reciprocating pilots can't ride, period. Encouraging pilots to "just give it a try" when they know they aren't allowed is incredibly selfish and irresponsibile. Now if an airline for some reason wants to allow non reciprocating pilots to JS, fine but if an airline doesn't, don't try, hoping to "get by" the agent and CA. That's very uncool. And at all airlines no one should ever, ever, attempt to jumpseat when they should be on a paid seat from their company.