It is not the sympathy you should challenge but the argument behind the sympathy. When someone tells you that you are being screwed and it is not something you already knew then you should be dubious about that claim because everyone is inherently tuned to noticing that kind of thing. It is a common tactic for example to pit two of your enemies against each other by convincing one that the other is mistreating them. For example you could "innocently" ask why the other is always giving orders in order to create a power struggle.

When someone you don't know tells you that you are being mistreated it is far more likely that they are trying to use you for their own purposes. In the case of communists they are trying to turn people against their government. It doesn't actually matter if anyone is being mistreated, simply that they believe it to be the case.