I think the difference is quite obvious. Much like taxes, dues are mandatory. You cannot opt out of them.
In fact, unions are unique in that sense, in that they have essentially the power to tax in the way that governments do. That is also the reason why we draw a similar analogy when we talk about why transparency requirements incumbent upon governments should also be imposed on unions: because they have a special power that no businesses—which obviously must get customers in order to generate revenue—or charities, or non-profits have.
A group like ours receives no mandatory money. If we do not keep our donors happy, we go out of business. That is not the case for governments, and it's not the case for unions.