Windows or Mac?
I use ctrl-c to copy data from FileMaker on a daily basis and this simply does not happen. It is not the default behavior to "replace the field contents with control characters". I can copy data, for example from a FileMaker Pro database field to paste here into the forum and I do this frequently.
You'll need to investigate more carefully to see why this is not happening in your system. Check to see if this is true for a field created in a brand new test file. That can rule out whether a script trigger, for example has changed the expected behavior in your original file and tells you that some setting in your OS would be the most likely culprit here.