Correct. Most video capture devices only can be accessed by one program at a time. Each Video Capture Device source counts as a separate 'program'.
There are a few ways around this though.
1) If you just need a simple crop, hold ALT and resize the source in the scene. This will only crop for that one instance.
2) If you need a more complex crop, or to use an Image Mask other Filters, add the existing video capture device to a Group, then apply filters to the group instead.
3) If you want to use the same filters in more than one scene, but still want access to the base unfiltered camera elsewhere, create a Scene and add your camera to it. Then add the filters to the scene, and add the scene to other scenes as a source.