I guess it depends on how they phrased it? I was only allowed so much sick time per year, which is pretty typical with US employers, and if I'd used it up I would not have any more paid sick days - no matter what I'd used it on. (In my case, I'd banked tons of sick leave and didn't have a problem, but still.) Actually, offering you 7 weeks of sick leave at all sounds very generous.
On the other hand, if they're talking about limiting your short-term disability eligibility or messing with FMLA (which is unpaid)... that might not be legal.