You are right. And getting fired can happen to anyone even if you follow the rules. Although the odds of getting fired if you follow the rules your employer has are lower than if you ignore them.Didn't the CM do it to themselves? If you steal from me, and I report you and you get arrested, it's not my fault, it's yours. Same thing here. The CM was wrong. Not the guest.
The biggest problem however is that Disney management has never given any indication that they really want the rules followed. If they simply watched a few rides on CCTV and removed any line jumpers in a very public fashion word would quickly get out that it wasn't tolerated and people would stop... but until that happens what's the incentive for any CM to do anything... Management has signaled that it doesn't really matter to them, so why would you expect a CM to care? In fact given a CM can easily get written up simply from a guest complaining against them with no hard evidence of any real act by the CM why would a CM risk upsetting a guest that might make up accusations against them to get them fired? Line jumping should be dealt with but the best way to deal with it isn't with a CM but with security that could easily use the CCTV cameras to combat the problem.
In fact I'm not really sure what security even does, I know when my wife told staff that a woman was stealing things from one of the shops at Hollywood Studios they did nothing and pretty much brushed it off as a cost of doing business.