That is by design, and yes I agree it is a silly restriction. It is my number one enhancement request. I often use variables to control the documents path in a WF that the user does not need to see in the data card, however, this restriction makes it so. Of course there are ways of hiding the variable in the data card so it doesn't show, but in my opinion an annoying work around.
This is a silly restriction and one that has been in the software since day 1 when it was called Conisio. There are 2 ways we work through this problem.
1. Make the variable read-only on the data card so that no one can mess with it.
2. Put control logic on the card to grey it out or hide it completely. Even if it is hidden or greyed out, the system will still write a property to it, and therefore transfer it to your drawing or other file through custom properties and attributes.