I've filed a ticket for this a while back. Somewhere in the past year the rendering of non editable checkboxes changed from greyed out checkboxes to labels. I solved it with images and conditional visbility (crappy). Ideally you would like to choose how to render non-editable checkboxes.
The reaction I got back then:
The issue has not been accepted as result of a design decision.
The value of non-editable fields are always shown as plain text, instead of as a disabled input field. A non-editable checkbox will show the system text as configured in the Modeler corresponding to the value of the boolean. One reason for this is that it is more clear that the field can not be edited. Another reason is that it is hard to see the value of a disabled - and thus grayed-out - checkbox.