It is certainly a gray area. The law talks about "manipulation of hours" and the fact that the employee is entitled to return to work with the same privileges. I think the fact that the employer discourages the employees FMLA use at all, would be considered a prohibited act.
My boss has threatened to change my days off because I often use my intermittent FMLA leave on my 5th day of work in a row. I have Rheumatoid Athritis, which affects my hands and hips. I am a casino card dealer. By my 3rd day of work my hands hurt so much that I need pain killers to get by...